https://bugs.gentoo.org/407591
Check for NULL pointer in response from conversation function.

--- pam_skey-1.1.5/pam_skey.c
+++ pam_skey/pam_skey.c
@@ -266,5 +266,8 @@
   else
     return PAM_SERVICE_ERR;
 
+  if (*response == NULL)
+    return PAM_SERVICE_ERR;
+
   return PAM_SUCCESS;
 }