malloc: Fix a realloc crash with heap tagging [BZ 27468]

_int_free must be called with a chunk that has its tag reset. This was
missing in a rare case that could crash when heap tagging is enabled:
when in a multi-threaded process the current arena runs out of memory
during realloc, but another arena still has space to finish the realloc
then _int_free was called without clearing the user allocation tags.

Fixes bug 27468.

Reviewed-by: DJ Delorie <dj@redhat.com>
(cherry picked from commit 42cc96066b)

malloc/malloc.c

@@ -3446,7 +3446,9 @@ __libc_realloc (void *oldmem, size_t bytes)
       newp = __libc_malloc (bytes);
       if (newp != NULL)
         {
-          memcpy (newp, oldmem, oldsize - SIZE_SZ);
+	  size_t sz = CHUNK_AVAILABLE_SIZE (oldp) - CHUNK_HDR_SZ;
+	  memcpy (newp, oldmem, sz);
+	  (void) TAG_REGION (chunk2rawmem (oldp), sz);
           _int_free (ar_ptr, oldp, 0);
         }
     }