returns truncated results

Problem: performig conversion from {INT, DECIMAL, REAL} to CHAR
we incorrectly set its max length in some cases that may lead
to truncated results returned.

Fix: properly set CONVERT({INT, DECIMAL, REAL}, CHAR) result's
max length.


mysql-test/r/ctype_utf8.result:
  Fix for bug#36772: When using UTF8, CONVERT with GROUP BY
  returns truncated results
    - test result.
mysql-test/t/ctype_utf8.test:
  Fix for bug#36772: When using UTF8, CONVERT with GROUP BY
  returns truncated results
    - test case.
sql/item_timefunc.cc:
  Fix for bug#36772: When using UTF8, CONVERT with GROUP BY
  returns truncated results
    - calculating Item_char_typecast::max_length use initial
  argument's charset mbmaxlen instead of from_cs->mbmaxlen,
  as from_cs may differ in some case (see comment above).