avcodec/indeo5: Do not leave frame_type set to an invalid value

Fixes: null pointer dereference
Fixes: 5264/clusterfuzz-testcase-minimized-4621956621008896

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>