Variable character_set_results can legally be NULL (for "no conversion.")
This could result in a NULL deref that crashed the server.  Fixed.

(Although ran some additional precursory tests to see whether I could break
anything else, but no breakage so far.)


mysql-test/r/func_time.result:
  Bug#21913: DATE_FORMAT() Crashes mysql server if I use it through mysql-connector-j driver.
  
  Prove DATE_FORMAT() no longer crashes the server when character_set_results is
  NULL (which is a legal value and means, "no conversion").
mysql-test/t/func_time.test:
  Bug#21913: DATE_FORMAT() Crashes mysql server if I use it through mysql-connector-j driver.
  
  Prove DATE_FORMAT() no longer crashes the server when character_set_results is
  NULL (which is a legal value and means, "no conversion").
sql/sql_string.cc:
  Bug#21913: DATE_FORMAT() Crashes mysql server if I use it through mysql-connector-j driver.
  
  Avoid NULL deref in my_charset_same() -- if !to_cs, we won't need to compare
  because it is magic for, "no conversion."