[SPARK-4714][CORE]: Add checking info is null or not while having gotten info.syn

[SPARK-4714][CORE]: Add checking info is null or not while having gotten info.syn
in removeBlock()/ dropOldBlock()/ dropFromMemory()
all have the same logic:
1. info = blockInfo.get(id)
2. if (info != null)
3. info.synchronized
there may be a possibility that while one thread got info.lock while the previous thread already removed from blockinfo in info.lock.
but one thing in current code, That not check info is null or not, while getting info.lock to remove block, will not cause any errors.

I don't think that this extra check is necessary; check out my comment on the main pull request and see if you agree.

Even if we did want to add a check here, I think we want to check for `if(blockInfo.get(blockId).nonEmpty)`, since this branch handles the case where blocks have _not_ been removed already.