cpukit: Prevent error with disabled stack checker

When the stack checker is not enabled, the stack checker reporting
function can still be called. This prevents that call from performing a
null memory access in trying to find the high water mark if the stack
checker was never initialized.

This also introduces a test to ensure this call does not cause a crash.

Closes #4588

spec/build/testsuites/libtests/grp.yml

@@ -247,6 +247,8 @@ links:
   uid: stackchk
 - role: build-dependency
   uid: stackchk01
+- role: build-dependency
+  uid: stackchk02
 - role: build-dependency
   uid: stat
 - role: build-dependency

spec/build/testsuites/libtests/stackchk02.yml

@@ -0,0 +1,19 @@
+SPDX-License-Identifier: CC-BY-SA-4.0 OR BSD-2-Clause
+build-type: test-program
+cflags: []
+copyrights:
+- Copyright (C) 2022 On-Line Applications Research (OAR)
+cppflags: []
+cxxflags: []
+enabled-by: true
+features: c cprogram
+includes: []
+ldflags: []
+links: []
+source:
+- testsuites/libtests/stackchk02/init.c
+stlib: []
+target: testsuites/libtests/stackchk02.exe
+type: build
+use-after: []
+use-before: []