c11/threads: Assert that mtx is non-NULL and check return values.

c11/threads: Assert that mtx is non-NULL and check return values.

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 <t_arceri@yahoo.com.au>
Reviewed-by: Brian Paul <brianp@vmware.com>