Attempt to perform arithmetic on a nil value within parser:parse() call

[Antwy]

Hello,

I was playing with libFuzzer-based Lua fuzzer called luzer and found crash with parser:parse() call.
Crash occurs when loading crash-4348634f6b8a3cb546a334528172d17549e49a03.txt file. You can use docker and fuzz targets from oss-sydr-fuzz to reproduce error:

/fuzz/stdin_parse_xml.lua < crash-00392e7545ad23ce26c768fafa8c14291b91eec0.txt

Stacktrace output:

lua: /usr/local/share/lua/5.1/XmlParser.lua:303: attempt to perform arithmetic on field 'extEnd' (a nil value)
stack traceback:
	/usr/local/share/lua/5.1/XmlParser.lua:303: in function 'parseNormalTag'
	/usr/local/share/lua/5.1/XmlParser.lua:356: in function 'parseTagType'
	/usr/local/share/lua/5.1/XmlParser.lua:428: in function 'parse'
	./oss/stdin_parse_xml.lua:12: in function 'TestOneInput'
	./oss/stdin_parse_xml.lua:17: in main chunk
	[C]: ?