libpthread: Remove unneeded NULL checks from semaphore functions

libpthread: Remove unneeded NULL checks from semaphore functions

Failing gracefully rather than crashing when passed a bogus sem_t
value is arguably an anti-feature, because it can mask mistakes in
programs that don't check return values.

glibc's semaphore implementation doesn't have these NULL checks.
POSIX doesn't require NULL checks here.

Remove tests that check for this error handling.

Also remove commented-out test code for checking interruption of
sem_wait() by an async signal.  NaCl is not likely to have
interruptible syscalls any time soon.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=715
TEST=run_semaphore_tests

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=11753

[Roland McGrath, 2013-07-12 22:35:28]

lgtm

[Mark Seaborn, 2013-07-13 01:10:23]

Committed patchset #1 manually as r11753 (presubmit successful).