Back out timegm error check from r272562.

POSIX treats negative time_t as undefined (i.e. may be valid too,
depends on system's policy we don't have) and we don't set EOVERFLOW
in mktime/timegm as POSIX requires to surely distinguish -1 return
as valid negative time from -1 as error return.

lib/libc/stdtime/strptime.c

@@ -676,8 +676,6 @@ strptime_l(const char * __restrict buf, const char * __restrict fmt,
 	if (ret && gmt) {
 		time_t t = timegm(tm);
 
-		if (t == -1)
-			return (NULL);
 		localtime_r(&t, tm);
 	}