Fix read past allocated buffer when parsing charsets file. (Bug #6413)

strings/xml.c:
  Don't look for comment past end of buffer.

strings/xml.c

@@ -81,7 +81,7 @@ static int my_xml_scan(MY_XML_PARSER *p,MY_XML_ATTR *a)
   a->beg=p->cur;
   a->end=p->cur;
   
-  if (!bcmp(p->cur,"<!--",4))
+  if ((p->end - p->cur > 3) && !bcmp(p->cur,"<!--",4))
   {
     for( ; (p->cur < p->end) && bcmp(p->cur, "-->", 3); p->cur++)
     {}