Fix yield nothing and yield yield parsing (345855, r=mrbkap).

js/src/jsparse.c

@@ -2009,7 +2009,11 @@ ReturnOrYield(JSContext *cx, JSTokenStream *ts, JSTreeContext *tc,
     if (tt2 == TOK_ERROR)
         return NULL;
 
-    if (tt2 != TOK_EOF && tt2 != TOK_EOL && tt2 != TOK_SEMI && tt2 != TOK_RC) {
+    if (tt2 != TOK_EOF && tt2 != TOK_EOL && tt2 != TOK_SEMI && tt2 != TOK_RC
+#if JS_HAS_GENERATORS
+        && (tt != TOK_YIELD || (tt2 != tt && tt2 != TOK_RB && tt2 != TOK_RP))
+#endif
+        ) {
         pn2 = operandParser(cx, ts, tc);
         if (!pn2)
             return NULL;