COMPMID-617: Adds validation to CLPoolingLayer

Change-Id: Ied405a9c0e9746598d03ac6a944ad87e9b6494eb
Reviewed-on: http://mpd-gerrit.cambridge.arm.com/93680
Tested-by: Kaizen <jeremy.johnson+kaizengerrit@arm.com>
Reviewed-by: Anthony Barbier <anthony.barbier@arm.com>
diff --git a/src/core/Utils.cpp b/src/core/Utils.cpp
index 0a35e07..bd6911f 100644
--- a/src/core/Utils.cpp
+++ b/src/core/Utils.cpp
@@ -384,4 +384,5 @@
         default:
             ARM_COMPUTE_ERROR("Undefined element size for given data type");
     }
+    return 0;
 }