chunky parser: only rewind if needed

The code reading chunked encoding attempts to rewind the code if it had
read more data than the chunky parser consumes. The rewinding can fail
and it will then cause an error. This change now makes the rewinding
only happen if pipelining is in use - as that's the only time it really
needs to be done.

Bug: http://curl.haxx.se/mail/lib-2010-08/0297.html
Reported by: Ron Parker