Fix buffer corruption in ReadUntil

[heyitsanthony]

This fixes two bugs:

1. join initialization prepends a nul character to the return string
2. the call to PutBack puts extra bytes back into the buffer

[ppalucki]

you may be interested in back-porting this one coreos#4 from coreos/gexpect fork

if fixes our problem we get from ReadUnitil if there are multbyte unicode characters

It looks like simiar issue - forgive me, I didn't made better analysis is it the same cause or not

[heyitsanthony]

Won't fix this issue; the slice handling here is broken even for single-byte runes.

[ThomasRooney]

Thanks! Sorry for the delay in merging