Demote modulo operations to integer if both args are int (456934, r=brendan).

js/src/jsbuiltins.cpp

@@ -86,15 +86,13 @@ js_dmod(jsdouble a, jsdouble b)
     return r;
 }
 
-/*
- * Note: Caller is responsible for ensuring that b is not 0, or really bad 
- *       things are going to happen.
- */
-
 jsint FASTCALL
 js_imod(jsint a, jsint b)
 {
-    return a % b;
+    if (a < 0 || b <= 0)
+        return -1;
+    int r = a % b;
+    return r;
 }
 
 /* The following boxing/unboxing primitives we can't emit inline because