sr: fx2lafw: Fix memory leaks in receive_transfer

sr: fx2lafw: Fix memory leaks in receive_transfer

There are a few memory leaks in the receive_transfer transfer function. The most
serve of them is that a sample buffer is not freed if the triggered has not
matched yet, which causes a sigrok process which is waiting for a trigger to
consume several megabytes of memory within seconds. The other leaks are on the
error paths in that function.

Signed-off-by: Lars-Peter Clausen <redacted>