Tweak reader_pid check

Check again after acquiring rmutex. Avoids potential issue with
a duplicate pid coming in between initial check and rmutex.

libraries/liblmdb/mdb.c

@@ -8092,11 +8092,13 @@ int mdb_reader_check(MDB_env *env, int *dead)
 			if (mdb_pid_insert(pids, pid) == 0) {
 				if (mdb_reader_pid(env, Pidcheck, pid)) {
 					LOCK_MUTEX_R(env);
+					if (mdb_reader_pid(env, Pidcheck, pid)) {
 						for (j=i; j<rdrs; j++)
 							if (mr[j].mr_pid == pid) {
 								mr[j].mr_pid = 0;
 								count++;
 							}
+					}
 					UNLOCK_MUTEX_R(env);
 				}
 			}