Resolve some compiler warnings (#328)

* build: resolve 2 instances of -Wpessimizing-move

clang 19 on FreeBSD 14.3 warns:

```
/usr/bin/c++ -DDEBUG -Dwmime_EXPORTS -I/home/ej/vmime -I/home/ej/vmime/src
-I/usr/local/include -D_REENTRANT=1 -W -Wall -pedantic
-Warray-bounds-pointer-arithmetic -Wold-style-cast -Wconversion -Wcast-align
-Wno-sign-conversion  -fvisibility=hidden -fvisibility-inlines-hidden -O0 -g
-std=c++17 -fPIC -DVMIME_SHARED -MD -MT
CMakeFiles/wmime.dir/src/vmime/net/imap/IMAPCommand.cpp.o -MF
CMakeFiles/wmime.dir/src/vmime/net/imap/IMAPCommand.cpp.o.d -o
CMakeFiles/wmime.dir/src/vmime/net/imap/IMAPCommand.cpp.o -c
/home/ej/vmime/src/vmime/net/imap/IMAPCommand.cpp

In file included from IMAPCommand.cpp:31:
In file included from IMAPConnection.hpp:40:
IMAPParser.hpp:3715:20: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
 | items.push_back(std::move(std::unique_ptr <msg_att_item>(parser.get <msg_att_item>(line, &pos))));
IMAPParser.hpp:3715:20: note: remove std::move call here
 | items.push_back(std::move(std::unique_ptr <msg_att_item>(parser.get <msg_att_item>(line, &pos))));
IMAPParser.hpp:4504:6: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
 | std::move(
IMAPParser.hpp:4504:6: note: remove std::move call here
 | std::move(
```

std::unique_ptr<T>(expr) is already an rvalue; invoking move() is
just redundant there.

* build: resolve 1 instance of -Wunused-but-set-variable

clang 19 on FreeBSD 14.3 warns:

```
In file included from IMAPCommand.cpp:31:
In file included from IMAPConnection.hpp:40:
IMAPParser.hpp:959:11: warning: variable 'len' set but not used [-Wunused-but-set-variable]
 | size_t len = 0;
```

* build: resolve 2 instances of -Wimplicit-int-conversion

clang 19 on FreeBSD 14.3 warns:

```
IMAPMessage.cpp:112:11: warning: higher order bits are zeroes after implicit conversion [-Wimplicit-int-conversion]
  112 |           m_size(-1U),
      |                 ~^~~
IMAPMessage.cpp:183:16: warning: higher order bits are zeroes after implicit conversion [-Wimplicit-int-conversion]
  183 |         if (m_size == -1U) {
      |                    ~~ ^~~
```

-1U is UINT_MAX; assigning that to size_t does not change the value
(stays UINT_MAX, does not become SIZE_MAX).

What is really intended here is ``size_t(-1)`` (or syntactically
equivalents). This is equal to ``size_t(0) - 1`` as per
https://en.cppreference.com/w/c/language/conversion.html § Integer
conversions ("repeatedly subtracted"), and because unsigned integers
implement modulo arithmetic, we get SIZE_MAX as desired.

Author: jengelh

src/vmime/net/imap/IMAPMessage.cpp

@@ -109,7 +109,7 @@ IMAPMessage::IMAPMessage(
 )
 	: m_folder(folder),
 	  m_num(num),
-	  m_size(-1U),
+	  m_size(-1),
 	  m_flags(FLAG_UNDEFINED),
 	  m_expunged(false),
 	  m_modseq(0),
@@ -180,7 +180,7 @@ vmime_uint64 IMAPMessage::getModSequence() const {
 
 size_t IMAPMessage::getSize() const {
 
-	if (m_size == -1U) {
+	if (m_size == static_cast<size_t>(-1)) {
 		throw exceptions::unfetched_object();
 	}
 

src/vmime/net/imap/IMAPParser.hpp

@@ -956,7 +956,6 @@ class VMIME_EXPORT IMAPParser : public object {
 		bool parseImpl(IMAPParser& parser, string& line, size_t* currentPos) {
 
 			size_t pos = *currentPos;
-			size_t len = 0;
 			bool valid = false;
 
 			value.reserve(line.length() - pos);
@@ -977,7 +976,6 @@ class VMIME_EXPORT IMAPParser : public object {
 					quoted = false;
 
 					++pos;
-					++len;
 
 				} else {
 
@@ -986,7 +984,6 @@ class VMIME_EXPORT IMAPParser : public object {
 						quoted = true;
 
 						++pos;
-						++len;
 
 					} else if (c == '"') {
 
@@ -999,7 +996,6 @@ class VMIME_EXPORT IMAPParser : public object {
 						value += c;
 
 						++pos;
-						++len;
 
 					} else {
 
@@ -3712,7 +3708,7 @@ class VMIME_EXPORT IMAPParser : public object {
 
 			VIMAP_PARSER_CHECK(one_char <'('> );
 
-			items.push_back(std::move(std::unique_ptr <msg_att_item>(parser.get <msg_att_item>(line, &pos))));
+			items.push_back(std::unique_ptr <msg_att_item>(parser.get <msg_att_item>(line, &pos)));
 
 			while (!VIMAP_PARSER_TRY_CHECK(one_char <')'> )) {
 				VIMAP_PARSER_CHECK(SPACE);
@@ -4501,9 +4497,7 @@ class VMIME_EXPORT IMAPParser : public object {
 			while ((resp = parser.get <IMAPParser::continue_req_or_response_data>(curLine, &pos))) {
 
 				continue_req_or_response_data.push_back(
-					std::move(
-						std::unique_ptr <IMAPParser::continue_req_or_response_data>(resp)
-					)
+					std::unique_ptr <IMAPParser::continue_req_or_response_data>(resp)
 				);
 
 				// Partial response (continue_req)