Only handle signals if we still have a VUT

This can happen if a signal is caught after `VUT_Fini` was called, but
at this point we should no longer manipulate the VUT being dismantled.

Original test case by Geoff, slightly modified.

Closes #2650

bin/varnishhist/varnishhist.c

@@ -494,7 +494,9 @@ profile_error(const char *s)
 static void
 vut_sighandler(int sig)
 {
-	VUT_Signaled(vut, sig);
+
+	if (vut != NULL)
+		VUT_Signaled(vut, sig);
 }
 
 int

bin/varnishlog/varnishlog.c

@@ -124,8 +124,9 @@ sighup(struct VUT *v)
 static void
 vut_sighandler(int sig)
 {
-	CHECK_OBJ_NOTNULL(vut, VUT_MAGIC);
-	VUT_Signaled(vut, sig);
+
+	if (vut != NULL)
+		VUT_Signaled(vut, sig);
 }
 
 int

bin/varnishncsa/varnishncsa.c

@@ -1124,8 +1124,9 @@ sighup(struct VUT *v)
 static void
 vut_sighandler(int sig)
 {
-	CHECK_OBJ_NOTNULL(vut, VUT_MAGIC);
-	VUT_Signaled(vut, sig);
+
+	if (vut != NULL)
+		VUT_Signaled(vut, sig);
 }
 
 static char *

bin/varnishtest/tests/r02649.vtc

+varnishtest "Cleanly stop a VUT app via vtc process -stop"
+
+varnish v1 -vcl {
+	backend be {
+		.host = "${bad_backend}";
+	}
+} -start
+
+process p1 {
+	exec varnishncsa -n ${v1_name} -P ${tmpdir}/ncsa.pid -w ${tmpdir}/ncsa.log
+} -start
+
+delay 1
+
+process p1 -expect-exit 0 -stop -wait
+
+# Expect empty stderr output
+shell -match {^0\b} "wc -c ${tmpdir}/p1/stderr"

bin/varnishtop/varnishtop.c

@@ -195,8 +195,9 @@ sighup(struct VUT *v)
 static void
 vut_sighandler(int sig)
 {
-	CHECK_OBJ_NOTNULL(vut, VUT_MAGIC);
-	VUT_Signaled(vut, sig);
+
+	if (vut != NULL)
+		VUT_Signaled(vut, sig);
 }
 
 static void