avformat/mxfdec: Don't use wrong type of pointer

avformat/mxfdec: Don't use wrong type of pointer

If one of the two results of a ternary conditional is a pointer to void,
the type of the whole conditional operator is a pointer to void, even
when the other possible result is not a pointer to void. This loophole
in the type system has allowed mxf_read_local_tags to have a pointer of
type pointer to MXFMetadataSet that actually points to an MXFContext.

Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>