fix(search) advance stream position even if there is a zero length match

[basarat]

E.g. if someone search for \b in regex mode the length of the match is 0 and then in readToken it will call the token function However since pos does not change it errors out throw new Error("Mode " + mode.name + " failed to advance stream.");

After the fix

Seems to work fine 🌹

[marijnh]

Good idea. Merged as 21f1d0a