Micro-optimize hb_language_from_string()

As measured / improved by Benson Limketkai.

src/hb-common.cc

@@ -293,21 +293,21 @@ retry:
 hb_language_t
 hb_language_from_string (const char *str, int len)
 {
-  char strbuf[64];
-
   if (!str || !len || !*str)
     return HB_LANGUAGE_INVALID;
 
+  hb_language_item_t *item = NULL;
   if (len >= 0)
   {
     /* NUL-terminate it. */
+    char strbuf[64];
     len = MIN (len, (int) sizeof (strbuf) - 1);
     memcpy (strbuf, str, len);
     strbuf[len] = '\0';
-    str = strbuf;
+    item = lang_find_or_insert (strbuf);
   }
-
-  hb_language_item_t *item = lang_find_or_insert (str);
+  else
+    item = lang_find_or_insert (str);
 
   return likely (item) ? item->lang : HB_LANGUAGE_INVALID;
 }