Hide compiler warning.

Hide compiler warning.

In file included from ../src/lib/grok/context.h:29,
                 from ../src/lib/j2k_encoder.h:36,
                 from ../src/lib/grok_j2k_encoder_thread.cc:5:
In member function ‘void grk_plugin::Synch::unlink()’,
    inlined from ‘grk_plugin::Synch::Synch(const string&, const string&)’ at ../src/lib/grok/messenger.h:200:10:
../src/lib/grok/messenger.h:247:52: error: ‘this’ pointer is null [-Werror=nonnull]
  247 |                         getMessengerLogger()->error("Error unlinking semaphore %s: %s", sentSemName_.c_str(),
      |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  248 |                                                                                 strerror(errno));

on Ubuntu 22.04.