BUG/MAJOR: spoe: Don't try to get agent config during SPOP healthcheck

BUG/MAJOR: spoe: Don't try to get agent config during SPOP healthcheck

During SPOP healthchecks, a dummy appctx is used to create the HAPROXY-HELLO
frame and then to parse the AGENT-HELLO frame. No agent are attached to it. So
it is important to not rely on an agent during these stages. When HAPROXY-HELLO
frame is created, there is no problem, all accesses to an agent are
guarded. This is not true during the parsing of the AGENT-HELLO frame. Thus, it
is possible to crash HAProxy with a SPOA declaring the async or the pipelining
capability during a healthcheck.

This patch must be backported to 1.9 and 1.8.