Fixed newly introduced bug in rollup


client/mysqldump.c:
  Safer buffer allocation
  Removed wrong assert
mysql-test/r/olap.result:
  more tests
mysql-test/t/olap.test:
  more tests
sql/handler.cc:
  Simple cleanup
  Fixed wrong check for next digit (wrong debug output)
sql/item.cc:
  Replace shrink_to_length() with mark_as_const() as the former allowed one to do changes to the string
sql/item_sum.cc:
  Change reference to pointer
  Trivial optimzation of testing 'allways_null'
sql/mysqld.cc:
  Proper indentation of comment
sql/sql_select.cc:
  Fixed newly introduced bug in rollup
sql/sql_string.h:
  Remove not needed 'shrink_to_length()'
  Added 'mark_as_const()' to be used when one want to ensure that a string is not changed