avformat/rmdec: Do not pass mime type in rm_read_multi() to ff_rm_read_mdpr_codecdata()

Fixes: use after free()
Fixes: rmdec-crash-ffe85b4cab1597d1cfea6955705e53f1f5c8a362

Found-by: Paul Ch <paulcher@icloud.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a7e032a2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>