avformat/rmdec: Check for multiple audio_stream_info

Fixes: memleak
Fixes: 39166/clusterfuzz-testcase-minimized-ffmpeg_dem_IVR_fuzzer-5153276690038784

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


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 8fe3566b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>