openmp: Fix next variable initialization in cp_parser_omp_clause_linear [PR123128]

openmp: Fix next variable initialization in cp_parser_omp_clause_linear [PR123128]

Apparently clang++ emits error on int *p = ((unsigned long) 0); while g++
accepts it without any diagnostics even with -pedantic-errors -W -Wall.
Dunno which is right, anyway, I meant to initialize with NULL, not
UNKNOWN_LOCATION.

2025-12-15  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123128
	* parser.cc (cp_parser_omp_clause_linear): Initialize next to NULL
	rather than UNKNOWN_LOCATION.