maintenance: compare output of pthread functions for inequality with 0

maintenance: compare output of pthread functions for inequality with 0

The documentation for pthread_create and pthread_sigmask state that:

"On success, pthread_create() returns 0;
on error, it returns an error number"

As such, we ought to check for an error
by seeing if the output is not 0.

Checking for "less than" is a mistake
as the error code numbers can be greater than 0.

Signed-off-by: Seija <doremylover123@gmail.com>
Acked-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>