avcodec/tiff: Avoid loosing allocated geotag values

Fixes memleak
Fixes: 2076/clusterfuzz-testcase-minimized-6542640243802112

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
