[core] reset var if FAMMonitorDirectory() fails

do not read fam_dir->version if FAMMonitorDirectory() fails

src/stat_cache.c

@@ -343,6 +343,7 @@ static void stat_cache_fam_dir_monitor(server *srv, stat_cache_fam *scf, stat_ca
 					FamErrlist[FAMErrno]);
 
 			fam_dir_entry_free(&scf->fam, fam_dir);
+			fam_dir = NULL;
 		} else {
 			int osize = splaytree_size(scf->dirs);