avcodec/utils: Clear dimensions in ff_get_buffer() on failure

Fixes out of array access
Fixes: 482d8f2fd17c9f532b586458a33f267c/asan_heap-oob_4a52b6_7417_1d08d477736d66cdadd833d146bb8bae.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>