BUG/MEDIUM: capture: capture.{req,res}.* crash without a stream

BUG/MEDIUM: capture: capture.{req,res}.* crash without a stream

Since commit 5f940703b3 ("MINOR: log: Don't depends on a stream to process
samples in log-format string") it has become quite obvious that a few sample
fetch functions and converters were still heavily dependent on the presence
of a stream without testing for it.

The capture.req.hdr, capture.res.hdr, capture.req.method, capture.req.uri,
capture.req.ver and capture.res.ver sample fetches used to assume the
presence of a stream, which is not necessarily the case (especially after
the commit above) and would crash haproxy if incorrectly used. Let's make
sure they check for this stream.

This fix adds a check for the stream's existence, and should be backported
to all stable versions up to 1.6.