Don't crash when calling ShConstructCompiler with a wrong output

BUG=angleproject:446

Change-Id: Ic1c326f79105950e5c3eb8724eba12fca187a2ec
Reviewed-on: https://chromium-review.googlesource.com/316610
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/compiler/translator/CodeGen.cpp b/src/compiler/translator/CodeGen.cpp
index 4f8d2b0..2b376d7 100644
--- a/src/compiler/translator/CodeGen.cpp
+++ b/src/compiler/translator/CodeGen.cpp
@@ -40,11 +40,11 @@
 #else
         // This compiler is not supported in this
         // configuration. Return NULL per the ShConstructCompiler API.
-        return NULL;
+        return nullptr;
 #endif // ANGLE_ENABLE_HLSL
       default:
         // Unknown format. Return NULL per the ShConstructCompiler API.
-        return NULL;
+        return nullptr;
     }
 }