Fix a schema type duration comparison overflow

https://bugzilla.gnome.org/show_bug.cgi?id=653724

Based on the fix suggested by Nick Pope <nick@nickpope.me.uk>
but just changing the casts to avoid using long long type
diff --git a/xmlschemastypes.c b/xmlschemastypes.c
index 1a5454c..834b261 100644
--- a/xmlschemastypes.c
+++ b/xmlschemastypes.c
@@ -3569,8 +3569,8 @@
 
     /* seconds */
     sec = x->value.dur.sec - y->value.dur.sec;
-    carry = (long)sec / SECS_PER_DAY;
-    sec -= (double)(carry * SECS_PER_DAY);
+    carry = (long)(sec / SECS_PER_DAY);
+    sec -= ((double)carry) * SECS_PER_DAY;
 
     /* days */
     day = x->value.dur.day - y->value.dur.day + carry;