Make sure the panic string is empty

bin/varnishd/cache/cache_panic.c

@@ -700,5 +700,6 @@ PAN_Init(void)
 	VSB_printf(pan_vsb, "This is a test\n");
 	AZ(VSB_finish(pan_vsb));
 	VSB_clear(pan_vsb);
+	heritage.panic_str[0] = '\0';
 	CLI_AddFuncs(debug_cmds);
 }

bin/varnishtest/tests/v00010.vtc

@@ -39,6 +39,9 @@ varnish v1 -arg "-smalloc,1m" -vcl+backend {
 	}
 } -start
 
+varnish v1 -cliok "stop"
+varnish v1 -cliok "start"
+varnish v1 -expect MGT.child_panic == 0
 varnish v1 -cliok "param.set feature +no_coredump"
 
 client c1 {