BUG/MEDIUM: mux_pt: dereference the connection with care in mux_pt_wake()

BUG/MEDIUM: mux_pt: dereference the connection with care in mux_pt_wake()

mux_pt_wake() calls data->wake() which can return -1 indicating that the
connection was just destroyed. We need to check for this condition and
immediately exit in this case otherwise we dereference a just freed
connection. Note that this mainly happens on idle connections between
two HTTP requests. It can have random implications between requests as
it may lead a wrong connection's polling to be re-enabled or disabled
for example, especially with threads.

This patch must be backported to 1.8.