Fix JS_GetArrayElement to not return an empty handle on success

BUG=chromium:528376
R=ulan@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/1337433002 .
diff --git a/fpdfsdk/src/jsapi/fxjs_v8.cpp b/fpdfsdk/src/jsapi/fxjs_v8.cpp
index f2f2e55..1dd479a 100644
--- a/fpdfsdk/src/jsapi/fxjs_v8.cpp
+++ b/fpdfsdk/src/jsapi/fxjs_v8.cpp
@@ -743,7 +743,7 @@
   if (pArray.IsEmpty())
     return v8::Local<v8::Value>();
   v8::Local<v8::Value> val;
-  if (pArray->Get(pJSRuntime->GetCurrentContext(), index).ToLocal(&val))
+  if (!pArray->Get(pJSRuntime->GetCurrentContext(), index).ToLocal(&val))
     return v8::Local<v8::Value>();
   return val;
 }