pgsql: Fix bogus length calculation that could lead to crash if the

Log Message:
-----------
Fix bogus length calculation that could lead to crash if the string
happened to be right up against the end of memory, per report from
Matt Magoffin.  While at it, avoid useless multiple copying of string
by not depending on xmlStrncatNew.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        xml.c (r1.58 -> r1.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.58&r2=1.59)