Remove dead NULL checks from pthread code.

Remove dead NULL checks from pthread code.

GCC is removing these checks anyway because it knows the arguments
must be non-null, so leaving this code around is just confusing.

We know from experience that people were shipping code with locking
bugs because they weren't checking for error returns. Failing hard
like glibc does seems the better choice. (And it's what the checked
in code was already doing; this patch doesn't change that. It just
makes it more obvious that that's what's going on.)

Change-Id: I167c6d7c0a296822baf0cb9b43b97821eba7ab35