This has been back-ported from 6.0 as the problems proved to afflict 
5.1 as well.
The fix exposed two new bugs. They were reported as follows.
      
Bug no 52174: Sometimes wrong plan when reading a MAX value 
from non-NULL index
      
Bug no 52173: Reading NULL value from non-NULL index gives wrong 
result in embedded server 
      
Both bugs taken together affect a much smaller class of queries than #47762, 
so the fix stays for now.