lavc: copy subtitle_header when copying context.

Fix trac ticket #2489.

libavcodec/options.c

@@ -226,6 +226,7 @@ int avcodec_copy_context(AVCodecContext *dest, const AVCodecContext *src)
     alloc_and_copy_or_fail(intra_matrix, 64 * sizeof(int16_t), 0);
     alloc_and_copy_or_fail(inter_matrix, 64 * sizeof(int16_t), 0);
     alloc_and_copy_or_fail(rc_override,  src->rc_override_count * sizeof(*src->rc_override), 0);
+    alloc_and_copy_or_fail(subtitle_header, src->subtitle_header_size, 1);
 #undef alloc_and_copy_or_fail
 
     return 0;