Probable XML parser bug: GC fatal: Thing is getting more internal refs than refs.

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1559

Reported by @grubba

/home/grubba/src/Pike/7.3/src/gc.c:959: GC fatal:
**Thing is getting more internal refs than refs.
**Location: 372b84  Type: mapping  Refs: 1
**Got gc marker at 44735c: flags=0x0002, refs=1, weak=0, xrefs=0, saved=1,
frame=0
**Describing mapping:
Refs=1, next=372b70, prev=372bd4, flags=0x0, size=1, hashsize=1
Indices type field =  string
Values type field =  int
/home/grubba/src/Pike/7.3/src/mapping.c:893: Fatal error:
Pike was in GC stage 100 when this fatal occured:
check_mapping_for_destruct called in invalid pass inside gc.
Attempting to dump backlog (may fail)...
Backtrace at time of fatal:
Tree: parse_xml_callback("<","p",([]),0,mapping[1])
/home/grubba/src/Pike/7.3/build/sol8-no-dmalloc/test-install/pike/7.3.5/lib/modules/spider.so.XML:
    parse("<module name=''><module name='MIME'><docgroup
homogen-name='decode_
    base64' homogen-type='method'><method
name='decode_base64'>\n<arguments><a
    rgument
name='encoded_data'><type>"+[7800],@0,0)
/home/grubba/src/Pike/7.3/build/sol8-no-dmalloc/test-install/pike/7.3.5/lib/modules/Parser.pmod/XML.pmod/Tree.pmod:580:
    parse_input("<module name=''><module name='MIME'><docgroup
homogen-name='d
    ecode_base64' homogen-type='method'><method
name='decode_base64'>\n<argume
    nts><argument
name='encoded_data'><type><string/></type></ar"+[7780],0,0)
/home/grubba/src/Pike/7.3/build/sol8-no-dmalloc/test-install/pike/7.3.5/lib/modules/Parser.pmod/XML.pmod/Tree.pmod:612:

parse_file("MIME/mime.xml")
/home/grubba/src/Pike/7.3/bin/join-autodoc.pike:21:

main(123,({"/home/grubba/src/Pike/7.3/bin/join-autodoc.pike","sub_manual.x

ml","CommonLog/clf.xml","FrontBase/frontbase.xml","Gdbm/gdbmmod.xml","Gett

ext/gettext.xml","Gmp/mpz_glue.xml","Gmp/my_mpz_xor.xml",,,114}))