Problem: we don't evaluate given expression checking values of the
slow_query_log_file/general_log_file, don't check it for NULL.

Fix: evaluate the expression, check result returned.


mysql-test/r/log_state.result:
  Fix bug #31604: server crash when setting slow_query_log_file/global general_log_file variable
    - test result.
mysql-test/t/log_state.test:
  Fix bug #31604: server crash when setting slow_query_log_file/global general_log_file variable
    - test case.
sql/set_var.cc:
  Fix bug #31604: server crash when setting slow_query_log_file/global general_log_file variable
    - evaluate var->value calling val_str() method;
    - check for null values returned;
    - return proper errors if any.