check for errors returned by get_capture_buffer()

if get_capture_buffer() returns <0, there is an error.

CID:28218
check whether there is at least 1 requestbuffer

rather than checking whether there are not-none buffers.
this allows for negative numbers of requestbuffers.

CID:28217
unsigned comparision against <0



we don't want to know whether f->index is really unsigned, so we
need to suppress the coverity false-positive

CID:28219