Fixed RasPi builds of unit tests.

On RasPi with gcc 4.8.2, the asserts wouldn't compile when tests were enabled.
Enforced the assumption that ASSERT code is generated only in Debug builds.

src/ClientHandle.cpp

@@ -238,7 +238,9 @@ AString cClientHandle::FormatMessageType(bool ShouldAppendChatPrefixes, eMessage
 		}
 	}
 	ASSERT(!"Unhandled chat prefix type!");
-	return "";
+	#ifndef __clang__
+		return "";
+	#endif
 }