Fix warnings reported by -Wconversion-null (#603)

Fix error reporting in clFillImage as a result.

Signed-off-by: Kevin Petit <kevin.petit@arm.com>

test_conformance/generic_address_space/main.cpp

@@ -75,5 +75,5 @@ const int test_num = ARRAY_SIZE( test_list );
 
 int main(int argc, const char *argv[])
 {
-    return runTestHarness(argc, argv, test_num, test_list, false, false, NULL);
+    return runTestHarness(argc, argv, test_num, test_list, false, false, 0);
 }