Validating parser backtraces

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1645

Reported by Jonas Wallden jonasw@roxen.com

Test program:

string xml =
#"<?xml version='1.0'?>
<!DOCTYPE doc [
  <!ELEMENT doc ((x|y)+,x,z)>
  <!ELEMENT x EMPTY>
  <!ELEMENT y EMPTY>
  <!ELEMENT z EMPTY>
]>
<doc>
  <y/>
  <x/>
  <z/>
</doc>";


int main()
{
  object p = Parser.XML.Validating();
  p->parse(xml, lambda(mixed ...args) {
                  werror("args: %O\n", args);
                  return 0;
                });
  return 0;
}

Backtrace:

Attempt to call the string "+"
/home/jonasw/pike-sås/7.2/lib/modules/Parser.pmod/XML.pmod/Validating.pike:182:
__lambda_65591_1("")
/home/jonasw/pike-sås/7.2/lib/modules/Parser.pmod/XML.pmod/Validating.pike:79:
Element("doc")->accept_element("")
/home/jonasw/pike-sås/7.2/lib/modules/Parser.pmod/XML.pmod/Validating.pike:324:
    validate("",0,0,"\n
",mapping[1],@0=__lambda_65585_0,@1=({}))
/home/jonasw/pike-sås/7.2/lib/modules/Parser.pmod/XML.pmod/Validating.pike:
    parse("<?xml version='1.0'?>\n<!DOCTYPE doc [\n  <!ELEMENT doc
((x|y)+,x,z)>\n  <!ELEMENT x EMPTY>\n  <!ELEMENT y EM
    PTY>\n  <!ELEMENT z EMPTY>\n]>\n<doc>\n  <y/>\n  <x/>\n
<z/>\n</doc>",@2=validate,@0,@1)
/home/jonasw/pike-sås/7.2/lib/modules/Parser.pmod/XML.pmod/Validating.pike:375:
    parse("<?xml version='1.0'?>\n<!DOCTYPE doc [\n  <!ELEMENT doc
((x|y)+,x,z)>\n  <!ELEMENT x EMPTY>\n  <!ELEMENT y EM
    PTY>\n  <!ELEMENT z EMPTY>\n]>\n<doc>\n  <y/>\n  <x/>\n
<z/>\n</doc>",@0)
validate-test.pike:22: main()