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>
FFmpeg: Cherry-pick c94305ae23318c8956a30485cd5642829f4f16a9 to fix #15085