avcodec/vmnc: Check location before use

Fixes: runtime error: signed integer overflow: 65535 * 64256 cannot be represented in type 'int'
Fixes: 1717/clusterfuzz-testcase-minimized-5491696676634624

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