common: Fix crash in language chooser

A report in Red Hat bugzilla [1] shows for some reason the country is set to
NULL and this causes a crash when comparing against search terms. The solution
is to handle the NULL without crashing.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1685251

panels/common/cc-language-chooser.c

@@ -188,6 +188,9 @@ match_all (gchar       **words,
 {
         gchar **w;
 
+        if (str == NULL)
+                return FALSE;
+
         for (w = words; *w; ++w)
                 if (!strstr (str, *w))
                         return FALSE;