glx: Fix error message when no driverName is available

glx: Fix error message when no driverName is available

Just provide a "(null)" literal in case driverName is NULL.

  In file included from ../src/glx/dri3_glx.c:76:
  ../src/glx/dri3_glx.c: In function ‘dri3_create_screen’:
  ../src/glx/dri_common.h:70:36: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
     70 | #define CriticalErrorMessageF(...) dri_message(_LOADER_FATAL, __VA_ARGS__)
        |                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../src/glx/dri3_glx.c:1002:4: note: in expansion of macro ‘CriticalErrorMessageF’
   1002 |    CriticalErrorMessageF("failed to load driver: %s\n", driverName);
        |    ^~~~~~~~~~~~~~~~~~~~~
  ../src/glx/dri3_glx.c:1002:50: note: format string is defined here
   1002 |    CriticalErrorMessageF("failed to load driver: %s\n", driverName);
        |                                                  ^~
  cc1: some warnings being treated as errors

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>