fix strptime output for %C without %y

in this case, a potentially-uninitialized or unrelated existing value
in tm_year was being used. instead use 0 if %y was not present.

src/time/strptime.c

@@ -11,7 +11,7 @@ char *strptime(const char *restrict s, const char *restrict f, struct tm *restri
 	int i, w, neg, adj, min, range, *dest, dummy;
 	const char *ex;
 	size_t len;
-	int want_century = 0, century = 0;
+	int want_century = 0, century = 0, relyear = 0;
 	while (*f) {
 		if (*f != '%') {
 			if (isspace(*f)) for (; *s && isspace(*s); s++);
@@ -144,7 +144,7 @@ char *strptime(const char *restrict s, const char *restrict f, struct tm *restri
 			if (!s) return 0;
 			break;
 		case 'y':
-			dest = &tm->tm_year;
+			dest = &relyear;
 			w = 2;
 			want_century |= 1;
 			goto numeric_digits;
@@ -198,6 +198,7 @@ char *strptime(const char *restrict s, const char *restrict f, struct tm *restri
 		}
 	}
 	if (want_century) {
+		tm->tm_year = relyear;
 		if (want_century & 2) tm->tm_year += century * 100 - 1900;
 		else if (tm->tm_year <= 68) tm->tm_year += 100;
 	}