Use length-bounded string functions to compare GumboStringPiece data
Use length-bounded string functions to compare GumboStringPiece data

All the current call sites of the functions fixed in this commit were
actually null-terminated, but GumboStringPieces usually aren't, so using
string routines that require null-termination is asking for trouble.

craigbarnes/lua-gumbo,craigbarnes/lua-gumbo,craigbarnes/lua-gumbo