bladeRF-cli: Addressed 'uninitialized variable' warning

Removed the n == 0 short circuit check in favor of initializing min_i
and min_q. This should keep compilers happy and be a bit more readable.
bladeRF-cli: Fixed use of uninitialized variable in error path

There's no need to jump to the 'out' label if the 'rx_samples'
allocation fails; nothing else has been allocated or started yet.