libbladeRF: Multiple fixes for test_readback.c

Fixes potential memory leak by moving memory allocation after calls
that return immediately on failure. (CID1249696)

Return BLADERF_ERR_MEM if calloc() fails. Otherwise, we'd have a NULL
dereference on our hands.

samplerate_tx_backup was being populated with the RX module's value and
its failure message contained "RX" -- fixed these apparent copy-paste
errors.