fix strftime %y for negative years

commit 583ea83541 fixed the case where
tm_year is negative but the resulting year (offset by 1900) was still
positive, which is always the case for time_t values that fit in 32
bits, but not for arbitrary inputs.

based on an earlier patch by Julien Ramseier which was overlooked at
the time the previous fix was applied.

src/time/strftime.c

@@ -166,8 +166,8 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		item = T_FMT;
 		goto nl_strftime;
 	case 'y':
-		val = tm->tm_year % 100;
-		if (val<0) val += 100;
+		val = (tm->tm_year + 1900LL) % 100;
+		if (val < 0) val = -val;
 		goto number;
 	case 'Y':
 		val = tm->tm_year + 1900LL;