return no rows

The algorithm of determining the best key for loose index scan is doing a loop
over the available indexes and selects the one that has the best cost.
It retrieves the parameters of the current index into a set of variables.
If the cost of using the current index is lower than the best cost so far it 
copies these variables into another set of variables that contain the 
information for the best index so far.
After having checked all the indexes it uses these variables (outside of the 
index loop) to create the table read plan object instance.
The was a single omission : the key_infix/key_infix_len variables were used 
outside of the loop without being preserved in the loop for the best index 
so far.
This causes these variables to get overwritten by the next index(es) checked.
Fixed by adding variables to hold the data for the current index, passing 
the new variables to the function that assigns values to them and copying 
the new variables into the existing ones when selecting a new current best 
index.
To avoid further such problems moved the declarations of the variables used 
to keep information about the current index inside the loop's compound 
statement.

mysql-test/r/group_min_max.result:
  Bug #41610: test case
mysql-test/t/group_min_max.test:
  Bug #41610: test case
sql/opt_range.cc:
  Bug #41610: copy the infix data for the current best index