Bug 1542980 - fixed js shell compilation error. r=jonco

This is from Bug 1534967, and the code is wrapped with compiler option
`JS_MORE_DETERMINISTIC`, which is only enabled through
'--enable-more-deterministic' in shell configure.

Differential Revision: https://phabricator.services.mozilla.com/D26656

--HG--
extra : moz-landing-system : lando

js/src/vm/Iteration.cpp

@@ -524,8 +524,8 @@ static bool Snapshot(JSContext* cx, HandleObject pobj_, unsigned flags,
    * behaviors are technically correct to do.
    */
 
-  jsid* ids = props->begin();
-  size_t n = props->length();
+  jsid* ids = props.begin();
+  size_t n = props.length();
 
   RootedIdVector tmp(cx);
   if (!tmp.resize(n)) {