can lead to bad memory access

Problem: Field_bit is the only field which returns INT_RESULT
and doesn't have unsigned flag. As it's not a descendant of the 
Field_num, so using ((Field_num *) field_bit)->unsigned_flag may lead
to unpredictable results.

Fix: check the field type before casting.


mysql-test/r/type_bit.result:
  Fix for bug #42803: Field_bit does not have unsigned_flag field,
  can lead to bad memory access
    - test result.
mysql-test/t/type_bit.test:
  Fix for bug #42803: Field_bit does not have unsigned_flag field,
  can lead to bad memory access
    - test case.
sql/opt_range.cc:
  Fix for bug #42803: Field_bit does not have unsigned_flag field,
  can lead to bad memory access
    - don't cast to (Field_num *) Field_bit, as it's not a Field_num
  descendant and is always unsigned by nature.