Don't crash on bogus keys in login.defs if PAM is enabled

Without this patch, PAM enabled builds crash when encountering an
invalid key in login.defs or key overrides because of array overflows

To reproduce, simply
	useradd -K Windows=broken

Signed-off-by: Bernhard Rosenkränzer <bero@lindev.ch>
Signed-off-by: Serge Hallyn <serge@hallyn.com>

lib/getdef.c

@@ -148,6 +148,7 @@ static struct itemdef knowndef_table[] = {
 #ifdef USE_PAM
 	PAMDEFS
 #endif
+	{NULL, NULL}
 };
 
 #ifndef LOGINDEFS