use global variable sax_parser

[ZigzagAK]

When we use xmlua with openresty we see warning:

2022/12/09 12:15:51 [warn] 289839#289839: *218 [lua] _G write guard:12: __newindex(): writing a global Lua variable ('sax_parser') which may lead to race conditions between concurrent requests,
stack traceback:
lib/xmlua/xml-stream-sax-parser.lua:74: in function 'ensure_sax_parser'
lib/xmlua/xml-stream-sax-parser.lua:83: in function 'parse'
lua/globals/xml.lua:159: in function 'decode_xml'

Need to add 'local' keyword.

https://github.com/clear-code/xmlua/blob/master/xmlua/xml-stream-sax-parser.lua#L74