Item*) at opt_sum.cc:305
      
Queries applying MIN/MAX functions to indexed columns are
optimized to read directly from the index if all key parts
of the index preceding the aggregated key part are bound to
constants by the WHERE clause. A prefix length is also
produced, equal to the total length of the bound key
parts. If the aggregated column itself is bound to a
constant, however, it is also included in the prefix.

Such full search keys are read as closed intervals for
reasons beyond the scope of this bug. However, the procedure
missed one case where a key part meant for use as range
endpoint was being overwritten with a NULL value destined
for equality checking. In this case the key part was
overwritten but the range flag remained, causing open
interval reading to be performed.

Bug was fixed by adding more stringent checking to the
search key building procedure (matching_cond) and never
allow overwrites of range predicates with non-range
predicates.

An assertion was added to make sure open intervals are never
used with full search keys.