Fix sloppy mode arguments uninitialized value use

MemorySanitizer complained about uninitialized reads in
the indexed property code path in JS_GetPropertyValue()
with JS_CLASS_MAPPED_ARGUMENTS objects.

Author: bnoordhuis

quickjs.c

@@ -14774,6 +14774,8 @@ static JSValue js_build_mapped_arguments(JSContext *ctx, int argc,
     if (JS_IsException(val))
         return val;
     p = JS_VALUE_GET_OBJ(val);
+    p->u.array.u.values = NULL;
+    p->u.array.count = 0;
 
     /* add the length field (cannot fail) */
     pr = add_property(ctx, p, JS_ATOM_length,