The assertion in String::copy was added in order to avoid
valgrind errors when the destination was the same as the source.
Eased restriction to allow for the case when str == NULL.


mysql-test/r/func_set.result:
  Bug#45168: Test result
mysql-test/t/func_set.test:
  Bug#45168: Test case
sql/item_strfunc.cc:
  Bug#45168: Code cleanup and grammar correction in comment
sql/sql_string.cc:
  Bug#45168: Fix