avformat/mxfenc: add a return at the end of non-void functions

Fixes compilation with GCC 11 when configured with --disable-optimizations

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/mxfenc.c

@@ -507,6 +507,7 @@ static const MXFLocalTagPair* mxf_lookup_local_tag(int tag)
 
     // this assert can only be hit during development
     av_assert0(0 && "you forgot to add your new tag to mxf_local_tag_batch");
+    return NULL;
 }
 
 static void mxf_mark_tag_unused(MXFContext *mxf, int tag)