Allow NULL in pthread_mutex_lock/unlock.

The pthread_mutex_lock and pthread_mutex_unlock were allowed to
fail silently on L 32 bit devices when passed a NULL. We changed
this to a crash on 32 bit devices, but there are still games that make
these calls and are not likely to be updated. Therefore, once again
allow NULL to be passed in on 32 bit devices.

Bug: 19995172
(cherry picked from commit 511cfd9d)

Change-Id: I159a99a941cff94297ef3fffda7075f8ef1ae252