wt_status: fix signedness mismatch in strbuf_read call

wt_status: fix signedness mismatch in strbuf_read call

We call strbuf_read(), and want to know whether we got any
output. To do so, we assign the result to a size_t, and
check whether it is non-zero.

But strbuf_read returns a signed ssize_t. If it encounters
an error, it will return -1, and we'll end up treating this
the same as if we had gotten output. Instead, we can just
check whether our buffer has anything in it (which is what
we care about anyway, and is the same thing since we know
the buffer was empty to begin with).

Note that the "len" variable actually has two roles in this
function. Now that we've eliminated the first, we can push the
declaration closer to the point of use for the second one.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>