Make all fragment shader out variables require location layout qualifier

Make all fragment shader out variables require location layout
qualifier. Previously, the last variable did not need a location layout
qualifier if the previous variables had those.

TEST=angle_unittests
BUG=angleproject:1070

Change-Id: Ifb66ee52b811409f5278eaad330d6cd9b8ea059f
Reviewed-on: https://chromium-review.googlesource.com/287100
Tested-by: Kimmo Kinnunen <kkinnunen@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/ValidateOutputs.cpp b/src/compiler/translator/ValidateOutputs.cpp
index 614d790..6976309 100644
--- a/src/compiler/translator/ValidateOutputs.cpp
+++ b/src/compiler/translator/ValidateOutputs.cpp
@@ -32,12 +32,13 @@
     {
         const TType &type = symbol->getType();
         const int location = type.getLayoutQualifier().location;
+        const bool isUnspecifiedOutputLocation = location == -1;
 
-        if (mHasUnspecifiedOutputLocation)
+        if (mHasUnspecifiedOutputLocation || (isUnspecifiedOutputLocation && !mOutputMap.empty()))
         {
             error(symbol->getLine(), "must explicitly specify all locations when using multiple fragment outputs", name.c_str());
         }
-        else if (location == -1)
+        else if (isUnspecifiedOutputLocation)
         {
             mHasUnspecifiedOutputLocation = true;
         }