Bug 860745 - Check result of ScriptSource::substring. r=luke

js/src/ion/AsmJSLink.cpp

@@ -381,6 +381,8 @@ HandleDynamicLinkFailure(JSContext *cx, CallArgs args, AsmJSModule &module, Hand
 
     uint32_t length = info.bufEnd_ - info.bufStart_;
     Rooted<JSStableString*> src(cx, info.scriptSource_->substring(cx, info.bufStart_, info.bufEnd_));
+    if (!src)
+        return false;
     const jschar *chars = src->chars().get();
 
     RootedFunction fun(cx, NewFunction(cx, NullPtr(), NULL, 0, JSFunction::INTERPRETED,