Fix NULL dereference (CID#149)

This bug can never happen, as we never call with pppsz_longname == NULL
at the moment.

src/modules/modules.c

@@ -736,7 +736,8 @@ char ** module_GetModulesNamesForCapability( const char *psz_capability,
     if( !psz_ret || ( pppsz_longname && *pppsz_longname == NULL ) )
     {
         free( psz_ret );
-        free( *pppsz_longname );
+        if( pppsz_longname )
+            free( *pppsz_longname );
         *pppsz_longname = NULL;
         module_list_free (list);
         return NULL;