gthread: fix minor errno problem in GCond
The return value from g_cond_wait_until()
is calculated, based on the
value of errno
after reacquiring the mutex. This is a problem because
errno
can be overwritten in the case the mutex is contended (in which
case the slow-path code will re-enter the kernel).
Perform the calculation before reacquiring the mutex.
See merge request !958 (merged)
Trivial backport of !958 (merged) to glib-2-60
.