codec_tags: fix wrong buffer size

Obvious mistake. This reported 44 bytes more data than what was
available. Could cause out of bounds reads. Security researchers would
claim a major victory if they found something like this in more popular
software, and would create a website for it.

demux/codec_tags.c

@@ -75,7 +75,7 @@ static void map_audio_pcm_tag(struct mp_codec_params *c)
 
         // Compressed formats might use this.
         c->extradata += 22;
-        c->extradata_size += 22;
+        c->extradata_size -= 22;
     }
 
     int bits = c->bits_per_coded_sample;