thd->allow_sum_func was left 'true' after previous statement thus allowing
sum funcs to be present in conditions.

thd->allow_sum_func should be set to 0 for each query and each prepared
statement reinitialization. This is done in lex_start() and 
reset_stmt_for_execute().


sql/sql_lex.cc:
  Fix bug#13180 thd->allow_sum_func wasn't reset obefore query processing.
  thd->allow_sum_func is set to 0 in lex_start().
sql/sql_prepare.cc:
  Fix bug#13180 thd->allow_sum_func wasn't reset obefore query processing.
  thd->allow_sum_func is set to 0 in  reset_stmt_for_execute().
mysql-test/t/update.test:
  Test case for bug#13180 thd->allow_sum_funcs wasn't reset before query
  processing.
mysql-test/r/update.result:
  Test case for bug#13180 thd->allow_sum_funcs wasn't reset before query
  processing.