avformat/mxfdec: Fix memleak upon repeating tags

When parsing MXF encountering some tags leads to allocations. And when
these tags were encountered repeatedly, this could lead to memleaks,
because the pointer to the old data got simply overwritten with a
pointer to the new data (or to NULL on allocation failure). This has
been fixed.

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

libavformat/mxfdec.c

@@ -850,6 +850,7 @@ static int mxf_read_cryptographic_context(void *arg, AVIOContext *pb, int tag, i
 static int mxf_read_strong_ref_array(AVIOContext *pb, UID **refs, int *count)
 {
     *count = avio_rb32(pb);
+    av_free(*refs);
     *refs = av_calloc(*count, sizeof(UID));
     if (!*refs) {
         *count = 0;
@@ -903,10 +904,8 @@ static int mxf_read_content_storage(void *arg, AVIOContext *pb, int tag, int siz
     case 0x1901:
         if (mxf->packages_refs)
             av_log(mxf->fc, AV_LOG_VERBOSE, "Multiple packages_refs\n");
-        av_free(mxf->packages_refs);
         return mxf_read_strong_ref_array(pb, &mxf->packages_refs, &mxf->packages_count);
     case 0x1902:
-        av_free(mxf->essence_container_data_refs);
         return mxf_read_strong_ref_array(pb, &mxf->essence_container_data_refs, &mxf->essence_container_data_count);
     }
     return 0;