shell: validate get_animation_type parameter

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index c27ccc7..b3e1c4f 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -406,6 +406,9 @@
 static enum animation_type
 get_animation_type(char *animation)
 {
+	if (!animation)
+		return ANIMATION_NONE;
+
 	if (!strcmp("zoom", animation))
 		return ANIMATION_ZOOM;
 	else if (!strcmp("fade", animation))