pthread: allow NULL thread to compare equal.

- POSIX says the behavior for pthread_equal is undefined for
uninitialized arguments.
- However, gcc C++11 threads supports expects C++-compatible behavior,
that is, two uninitialized pthread_t should compare equal.

Avoids some runtime asserts in latest WebKit version.

src/system/libroot/posix/pthread/pthread.cpp

@@ -181,7 +181,7 @@ pthread_self(void)
 int
 pthread_equal(pthread_t t1, pthread_t t2)
 {
-	return t1 != NULL && t2 != NULL && t1 == t2;
+	return t1 == t2;
 }