flacdec: fix buffer size checking in get_metadata_size()

Adds an additional check before reading the next block header and avoids a
potential integer overflow when checking the metadata size against the
remaining buffer size.
(cherry picked from commit 4c5e7b27d5)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/flacdec.c

@@ -228,9 +228,11 @@ static int get_metadata_size(const uint8_t *buf, int buf_size)
 
     buf += 4;
     do {
+        if (buf_end - buf < 4)
+            return 0;
         ff_flac_parse_block_header(buf, &metadata_last, NULL, &metadata_size);
         buf += 4;
-        if (buf + metadata_size > buf_end) {
+        if (buf_end - buf < metadata_size) {
             /* need more data in order to read the complete header */
             return 0;
         }