Fix compiler warning: ISO C90 forbids mixed declarations and code

lib/memdebug.c

@@ -211,10 +211,10 @@ void *curl_dorealloc(void *ptr, size_t wantedsize,
 {
   struct memdebug *mem=NULL;
 
-  assert(wantedsize != 0);
-
   size_t size = sizeof(struct memdebug)+wantedsize;
 
+  assert(wantedsize != 0);
+
   if(countcheck("realloc", line, source))
     return NULL;