avcodec/gifdec: Add av_log() to workaround gcc 4.2 miscompiling the code

It appears gcc forgets to increment the byte stream pointer at -O3
I was not able to spot a reason/excuse for it to do that. It
also disappears if the function isnt inlined.

This should fix the gif fate failures on open & free bsd

See: [FFmpeg-devel] [PATCH] gifdec: workaround old gcc 4.2 mis-compiling code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>