golomb: Fix the implementation of get_se_golomb_long

This was only used in hevc muxing code so far.

This makes the return values match what get_se_golomb returns for
the same bitstream reader instances.

The logic for producing a signed golomb code out of an unsigned one
was based on the corresponding code in get_se_golomb, which operated
directly on the bitstream reader buffer - not on the equivalent
return value from get_ue_golomb.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>