Problem: LOGGER::general_log_write() relied on valid "thd" parameter passed
but had inconsistent "if (thd)" check.

Fix: as we always pass a valid "thd" parameter to the method, 
redundant check removed.


sql/log.cc:
  Fix for bug#47130: misplaced or redundant check for null pointer?
    - code clean-up, as we rely on the "thd" parameter in the
  LOGGER::general_log_write(), redundant "if (thd)" check removed, 
  added assert(thd) instead.