Crashes with comment only files

[jdesgats]

When a file contains only comments (use case: ldoc API definitions), the formatter crashes:

printf -- '-- this is just a comment\n-- more comments\n' | luaformatter /dev/stdin
/usr/bin/lua5.1: ...ome/julien/.luaenv/check/share/lua/5.1/formatter.lua:57: attempt to index local 'node' (a nil value)
stack traceback:
	...ome/julien/.luaenv/check/share/lua/5.1/formatter.lua:57: in function 'getlastline'
	...ome/julien/.luaenv/check/share/lua/5.1/formatter.lua:373: in function 'getindentlevel'
	...ome/julien/.luaenv/check/share/lua/5.1/formatter.lua:512: in function 'indentcode'
	...uarocks/rocks-5.1/formatter/0.1.1-1/bin/luaformatter:79: in main chunk
	[C]: ?

Interestingly, the issue doesn't exist with a single line.