MINOR: conn/muxes/ssl: add ASSUME_NONNULL() prior to _srv_add_idle

MINOR: conn/muxes/ssl: add ASSUME_NONNULL() prior to _srv_add_idle

When manipulating idle backend connections for input/output processing,
special care is taken to ensure the connection cannot be accessed by
another thread, for example via a takeover. When processing is over,
connection is reinserted in its original list.

A connection can either be attached to a session (private ones) or a
server idle tree. In the latter case, <srv> is guaranteed to be non null
prior to _srv_add_idle() thanks to CO_FL_LIST_MASK comparison with conn
flags. This patch adds an ASSUME_NONNULL() to better reflect this.

This should fix coverity reports from github issue #3095.