Rewrite blockdata_retrieve() and expand_buf() to use realloc().

src/util.c

@@ -680,18 +680,12 @@ int expand_buf(struct iovec *iov, size_t size)
   if (size <= (size_t)iov->iov_len)
     return 1;
 
-  if (!(new = whine_malloc(size)))
+  if (!(new = whine_realloc(iov->iov_base, size)))
     {
       errno = ENOMEM;
       return 0;
     }
 
-  if (iov->iov_base)
-    {
-      memcpy(new, iov->iov_base, iov->iov_len);
-      free(iov->iov_base);
-    }
-
   iov->iov_base = new;
   iov->iov_len = size;