avcodec/tiff: Update pointer only when the result is used

Fixes: runtime error: signed integer overflow: 538976288 * 32 cannot be represented in type 'int'
Fixes: 2310/clusterfuzz-testcase-minimized-4534784887881728

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