lib/test.sh: Remove temporary directory only if we know the location

This is a problem when a test TCONF before calling tst_tmpdir, but has
tst_rmdir in cleanup function (e.g. dhcp_lib.sh when failing on
tst_check_cmds).

Now it has the same behaviour as deleting $TST_TMPDIR in tst_do_exit
in tst_test.sh.

Signed-off-by: Petr Vorel <pvorel@suse.cz>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/testcases/lib/test.sh b/testcases/lib/test.sh
index 642f7ea..d16444e 100644
--- a/testcases/lib/test.sh
+++ b/testcases/lib/test.sh
@@ -132,8 +132,10 @@
 
 tst_rmdir()
 {
-	cd "$LTPROOT"
-	rm -r "$TST_TMPDIR"
+	if [ -n "$TST_TMPDIR" ]; then
+		cd "$LTPROOT"
+		rm -r "$TST_TMPDIR"
+	fi
 }
 
 #