Fixes timeout with 686/clusterfuzz-testcase-5853946876788736

this shortcuts (i.e. speeds up) the error and
return-to-user when decoding a truncated frame

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Previous version reviewed by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>