COMPMID-1253: Nightly: Fix Canny Edge NEON failing

Change-Id: If0836522792717a843c1cab405afc9320ce53079
Reviewed-on: https://eu-gerrit-1.euhpc.arm.com/137162
Tested-by: Jenkins <bsgcomp@arm.com>
Reviewed-by: Anthony Barbier <anthony.barbier@arm.com>
diff --git a/tests/validation/Helpers.cpp b/tests/validation/Helpers.cpp
index 521cc57..e2415a2 100644
--- a/tests/validation/Helpers.cpp
+++ b/tests/validation/Helpers.cpp
@@ -141,7 +141,7 @@
 
     params.constant_border_value = int_dist(gen);
     params.upper_thresh          = threshold_dist(gen); // upper_threshold >= 1
-    threshold_dist               = std::uniform_int_distribution<uint8_t>(0, params.upper_thresh);
+    threshold_dist               = std::uniform_int_distribution<uint8_t>(1, params.upper_thresh - 1);
     params.lower_thresh          = threshold_dist(gen);
 
     return params;