partially revert a change intended to alter the log message - fixes 4038

Log(LOG_LEVEL_VERBOSE, "Skipping next promise '%s', as context '%s' is not relevant", pp->promiser, pp->classes);

was changed to 

Log(LOG_LEVEL_VERBOSE, "Skipping next promise '%s', as var-context '%s' is not relevant", pp->promiser, sp);

by 4fa70ef078336f225740c17c7cdefb258d6a0ba3

When sp is NULL (if the classes list is empty), this results in segfault. If the change is partially reverted so that pp->classes is retained, the bug disappears.