ff_id3v2_free_extra_meta: set the pointer pointing to extra_meta to NULL

Fixes ticket #3530.
Signed-off-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/id3v2.c

@@ -937,6 +937,8 @@ void ff_id3v2_free_extra_meta(ID3v2ExtraMeta **extra_meta)
         av_freep(&current);
         current = next;
     }
+
+    *extra_meta = NULL;
 }
 
 int ff_id3v2_parse_apic(AVFormatContext *s, ID3v2ExtraMeta **extra_meta)