BUG/MINOR: http-fetch: Fix smp_fetch_body() when called from a health-check

BUG/MINOR: http-fetch: Fix smp_fetch_body() when called from a health-check

res.body may be called from a health-check. It is probably never used. But it is
possibe. In such case, there is no channel. Thus we must not use it
unconditionally to set the flag SMP_F_MAY_CHANGE on the smp.

Now the condition test the channel first. In addtion, the flag is not set if the
payload is fully received.

This patch must be backported as far as 2.2.

(cherry picked from commit a9ffc416377e0df9859526dc3c1d769c6a68636f)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>