Loose index scan using only second part of multipart index was choosen, which
results in creating wrong keys and endless loop.

get_best_group_min_max() now allows loose index scan for distinct only if used
keyparts forms a prefix of the index.


mysql-test/t/group_min_max.test:
  Test case for bug #13293 Wrongly used index results in endless loop.
mysql-test/r/group_min_max.result:
  Test case for bug #13293 Wrongly used index results in endless loop.
sql/opt_range.cc:
  Fix bug #13293 Wrongly used index results in endless loop.
  get_best_group_min_max() now allows loose index scan for distinct only if used keyparts forms a prefix of the index.