by item->val_str() may be a substring of the passed string. 
Disallow string=its_substring assignment in String::operator=().


mysql-test/r/func_misc.result:
  Testcase for BUG#7716
mysql-test/t/func_misc.test:
  Testcase for BUG#7716
sql/item_cmpfunc.cc:
  Fix for BUG#7716: in in_string::set() take into account that the string returned 
  by item->val_str(S) may be not S but use the buffer owned by S.
sql/sql_string.h:
  * Added assert: String& String::operator=(const String&) may not be used to do 
    assignments like str = string_that_uses_buffer_owned_by_str
  * Added String::uses_buffer_owned_by().