Fix a problem which may cause nullptr in ResultBufferMgr (#882)
Fix a problem which may cause nullptr in ResultBufferMgr (#882)

Because there is no lock in the middle call to findBlock(),
even if the first call returns non-null, the second call
may still get a null block
imay/palo,imay/palo,imay/palo,imay/palo,imay/palo,imay/palo