Fix potential SIOF issue with checker layers

Since the order of global initialization is undefined, the context
variable, accessed directly from the events and leak checkers, might
be unitialized.

This patch fixes this by using the getInstance() accessor method that
makes sure that the validation layer context is initialized before use.

Author: pbalcer

source/layers/validation/checkers/basic_leak/zel_basic_leak_checker.cpp

@@ -26,7 +26,7 @@ namespace validation_layer
             basic_leak_checker.zeValidation = zeChecker;
             basic_leak_checker.zetValidation = zetChecker;
             basic_leak_checker.zesValidation = zesChecker;
-            validation_layer::context.validationHandlers.push_back(&basic_leak_checker);
+            validation_layer::context.getInstance().validationHandlers.push_back(&basic_leak_checker);
         }
     }
 

source/layers/validation/checkers/events_checker/zel_events_checker.cpp

@@ -25,7 +25,7 @@ eventsChecker::eventsChecker() {
         events_checker.zesValidation = zesChecker;
         events_checker.zetValidation = zetChecker;
 
-        validation_layer::context.validationHandlers.push_back(&events_checker);
+        validation_layer::context.getInstance().validationHandlers.push_back(&events_checker);
     }
 }