bladeRF-cli: Addressed possible NULL-dereference (CID1250040)

The status here is forced to a non-zero value (CLI_RET_MEM) instead
of assuming *every* malloc implementation will set errno to ensure
we don't enter the loop and attempt to use tx_buffer when NULL.
bladeRF-cli: Changed type of tx_buffer from void* to int16_t*

There's no reason to use void* here. Using int16_t* to be more explicit,
since SC16Q11 samples are being used.