Check for tmon in _xmlSchemaDateAdd() is incorrect

For https://bugzilla.gnome.org/show_bug.cgi?id=732705
In _xmlSchemaDateAdd(), the check for |tmon| should be the following
since MAX_DAYINMONTH() expects a month in the range [1,12]:

    if (tmon < 1)
	tmon = 1;

Regression introduced in
https://git.gnome.org/browse/libxml2/commit/?id=14b5643947845df089376106517c4f7ba061e4b0
diff --git a/xmlschemastypes.c b/xmlschemastypes.c
index 03a55df..7a56267 100644
--- a/xmlschemastypes.c
+++ b/xmlschemastypes.c
@@ -3848,8 +3848,8 @@
 	     * Coverity detected an overrun in daysInMonth
 	     * of size 12 at position 12 with index variable "((r)->mon - 1)"
 	     */
-	    if (tmon < 0)
-	        tmon = 0;
+	    if (tmon < 1)
+	        tmon = 1;
 	    if (tmon > 12)
 	        tmon = 12;
             tempdays += MAX_DAYINMONTH(tyr, tmon);