[libxslt] Fix integer overflow in _exsltDateDayInWeek

[Nick Wellnhofer <nwellnhof src gnome org>]

commit c75b811de0afeea6acf19c99a755b8e1c0585aa9
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Thu Apr 25 11:16:58 2019 +0200

    Fix integer overflow in _exsltDateDayInWeek
    
    Found by OSS-Fuzz.

 libexslt/date.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/libexslt/date.c b/libexslt/date.c
index 32c9db70..d075adc5 100644
--- a/libexslt/date.c
+++ b/libexslt/date.c
@@ -1474,11 +1474,12 @@ _exsltDateDayInWeek(long yday, long yr)
     long ret;
 
     if (yr <= 0) {
-        ret = ((yr-2 + ((yr/4)-(yr/100)+(yr/400)) + yday) % 7);
+        /* Compute modulus twice to avoid integer overflow */
+        ret = ((yr%7-2 + ((yr/4)-(yr/100)+(yr/400)) + yday) % 7);
         if (ret < 0)
             ret += 7;
     } else
-        ret = (((yr-1) + (((yr-1)/4)-((yr-1)/100)+((yr-1)/400)) + yday) % 7);
+        ret = (((yr%7-1) + (((yr-1)/4)-((yr-1)/100)+((yr-1)/400)) + yday) % 7);
 
     return ret;
 }