Extensions & antenna mixin bug

[rkossler]

Issue Description

Calling function "usrp->set_rx_antenna()" with extensions interface causes "segmentation fault"

Setup Details

My setup included an N320 using head of UHD-4.9 (d589cd5), built from source, with default FPGA image and unmodified extension example

Expected Behavior

no segmentation fault

Actual Behaviour

segmentation fault

Steps to reproduce the problem

$ rx_samples_to_file --freq 2450e6 --gain 30 --nsamps 1000 --rate 30.72e6 --args="extension=extension_example" --ant RX2

Additional Information