PSYNC: safer handling of PSYNC requests.

There was a bug that over-esteemed the amount of backlog available,
however this could only happen when a slave was asking for an offset
that was in the "future" compared to the master replication backlog.

Now this case is handled well and logged as an incident in the master
log file.

src/replication.c

@@ -356,10 +356,14 @@ int masterTryPartialResynchronization(redisClient *c) {
        REDIS_OK) goto need_full_resync;
     if (!server.repl_backlog ||
         psync_offset < server.repl_backlog_off ||
-        psync_offset >= (server.repl_backlog_off + server.repl_backlog_size))
+        psync_offset > (server.repl_backlog_off + server.repl_backlog_histlen))
     {
         redisLog(REDIS_NOTICE,
             "Unable to partial resync with the slave for lack of backlog (Slave request was: %lld).", psync_offset);
+        if (psync_offset > server.master_repl_offset) {
+            redisLog(REDIS_WARNING,
+                "Warning: slave tried to PSYNC with an offset that is greater than the master replication offset.");
+        }
         goto need_full_resync;
     }