for each record'

There was an error in an internal structure in the range
optimizer (SEL_ARG). Bad design causes parts of a data
structure not to be initialized when it is in a certain
state. All client code must check that this state is not
present before trying to access the structure's data. Fixed
by

- Checking the state before trying to access data (in
several places, most of which not covered by test case.)

- Copying the keypart id when cloning SEL_ARGs


mysql-test/r/range.result:
  Bug#48459: Test result.
mysql-test/t/range.test:
  Bug#48459: Test case.
sql/opt_range.cc:
  Bug#48459: Fix + doxygenated count_key_part_usage comment.