avio_read may return EOF, leaving the mimetype array unitialized. fail
early when this occurs to avoid using the array in an unitialized state.
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ee1e39a5)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>