mktime_utc(): save and restore errno when using timegm().

At least two timegm() implementations, and possibly more, can change
errno even if they succeed.  Save and restore errno, so the "we have
timegm()" implementation of mktime_utc() leaves errno alone, just as the
"we don't have timegm()" implementation does.

This should address the problem mentioned in !11954.

wsutil/time_util.c

@@ -30,9 +30,8 @@
 time_t
 mktime_utc(struct tm *tm)
 {
-#ifndef HAVE_TIMEGM
 	time_t retval;
-
+#ifndef HAVE_TIMEGM
 	static const int days_before[] =
 		{
 			0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
@@ -57,7 +56,30 @@ mktime_utc(struct tm *tm)
 
 	return retval;
 #else
-	return timegm(tm);
+	int save_errno;
+
+	/*
+	 * If passed a struct tm for 2013-03-01 00:00:00, both
+	 * macOS and FreeBSD timegm() return the epoch time
+	 * value for 2013-03-01 00:00:00 UTC, but also set
+	 * errno to EOVERFLOW.  This may be true of other
+	 * implementations based on the tzcode reference
+	 * impelementation of timegm().
+	 *
+	 * The macOS and FreeBSD documentation for timegm() neither
+	 * commit to leaving errno alone nor commit to setting it
+	 * to a particular value.
+	 *
+	 * The code we use if we don't have timegm() doesn't
+	 * set errno.
+	 *
+	 * We save and restore errno, so our callers don't need to
+	 * worry about errno getting overwritten.
+	 */
+	save_errno = errno;
+	retval = timegm(tm);
+	errno = save_errno;
+	return retval;
 #endif /* !HAVE_TIMEGM */
 }