ucs2 doesn't provide required by fulltext ctype array. Crash
happens because fulltext attempts to use unitialized ctype
array.

Fixed by converting ucs2 fields to compatible utf8 analogue.


include/my_sys.h:
  Added a function to find compatible character set with ctype array
  available. Currently used by fulltext search to find compatible
  substitute for ucs2 collations.
mysql-test/r/ctype_ucs.result:
  A test case for BUG#31159.
mysql-test/t/ctype_ucs.test:
  A test case for BUG#31159.
mysys/charset.c:
  Added a function to find compatible character set with ctype array
  available. Currently used by fulltext search to find compatible
  substitute for ucs2 collations.
sql/item_func.cc:
  Convert ucs2 fields to utf8. Fulltext requires ctype array, but
  ucs2 doesn't provide it.