glsl/glcpp: fix SIGSEGV when checking error condition for macro redefinition

glsl/glcpp: fix SIGSEGV when checking error condition for macro redefinition

Commit a6e9cd14c does not take into account than node_{a,b}->next could be NULL
in some circumstances, such as in a shader containing this code:

  #define A 1 /* comment */
  #define A 1 /* comment */

This patch fixes the segmentation fault for cases like that.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91290
Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: mesa-stable@lists.freedesktop.org