COMPMID-2486: Remove/add disabled compiler warnings

Removed the following flags:
-Wno-vla
-Wno-strict-overflow

Added:
-Wformat-security

Change-Id: I49eb3d724e14db796e543164295674617c37cb65
Signed-off-by: Michalis Spyrou <michalis.spyrou@arm.com>
Reviewed-on: https://review.mlplatform.org/c/2109
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
diff --git a/tests/validation/reference/WarpAffine.cpp b/tests/validation/reference/WarpAffine.cpp
index 7b903b7..2a7aeb7 100644
--- a/tests/validation/reference/WarpAffine.cpp
+++ b/tests/validation/reference/WarpAffine.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017 ARM Limited.
+ * Copyright (c) 2017-2019 ARM Limited.
  *
  * SPDX-License-Identifier: MIT
  *
@@ -63,7 +63,8 @@
     const int width  = src.shape().x();
     const int height = src.shape().y();
 
-    for(int element_idx = 0; element_idx < src.num_elements(); ++element_idx)
+    const uint32_t num_elements = src.num_elements();
+    for(uint32_t element_idx = 0; element_idx < num_elements; ++element_idx)
     {
         valid_mask[element_idx] = 1;
         Coordinates id          = index2coord(src.shape(), element_idx);