Passing NULL to C11 threads functions isn't safe, so there's no need for
our implementation to handle it. Cuts about 1k of .text.
text data bss dec hex filename
5009514 198440 26328 5234282 4fde6a i965_dri.so before
5008346 198440 26328 5233114 4fd9da i965_dri.so after
Reviewed-by: Timothy Arceri <firstname.lastname@example.org>
Reviewed-by: Brian Paul <email@example.com>