Tweak mdb_envinfo numreaders

Return the actual shared reader count when it exists, not
just the current process env's reader count.

libraries/liblmdb/mdb.c

@@ -7682,7 +7682,12 @@ mdb_env_info(MDB_env *env, MDB_envinfo *arg)
 	arg->me_mapaddr = (env->me_flags & MDB_FIXEDMAP) ? env->me_map : 0;
 	arg->me_mapsize = env->me_mapsize;
 	arg->me_maxreaders = env->me_maxreaders;
-	arg->me_numreaders = env->me_numreaders;
+
+	/* me_numreaders may be zero if this process never used any readers. Use
+	 * the shared numreader count if it exists.
+	 */
+	arg->me_numreaders = env->me_txns ? env->me_txns->mti_numreaders : env->me_numreaders;
+
 	arg->me_last_pgno = env->me_metas[toggle]->mm_last_pg;
 	arg->me_last_txnid = env->me_metas[toggle]->mm_txnid;
 	return MDB_SUCCESS;