avformat/mxfdec: fix null pointer dereference

Fixes: signal_sigsegv_b5b3d4_2_001.mxf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -1457,7 +1457,7 @@ static int mxf_parse_physical_source_package(MXFContext *mxf, MXFTrack *source_t
             break;
 
         /* the name of physical source package is name of the reel or tape */
-        if (physical_package->name[0])
+        if (physical_package->name && physical_package->name[0])
             av_dict_set(&st->metadata, "reel_name", physical_package->name, 0);
 
         /* the source timecode is calculated by adding the start_position of the sourceclip from the file source package track