BUG/MEDIUM: server: srv_set_addr_desc() crashes when a server has no address

BUG/MEDIUM: server: srv_set_addr_desc() crashes when a server has no address

GitHub Issue #1026 reported a crash during configuration check for the
following example config:

    backend 0
    server 0 0
    server 0 0

HAProxy crashed in srv_set_addr_desc() due to a NULL pointer dereference
caused by `sa2str` returning NULL for an `AF_UNSPEC` address (`0`).

Check to make sure the address key is non-null before using it for
comparison or inserting it into the tree.

The crash was introduced in commit 92149f9a8 ("MEDIUM: stick-tables: Add
srvkey option to stick-table") which not in any released version so no
backport is needed.

Cc: Tim Duesterhus <tim@bastelstu.be>