CLEANUP: server: be sure never to compare src against a non-existing defsrv

CLEANUP: server: be sure never to compare src against a non-existing defsrv

The test in srv_ssl_settings_cpy() comparing src to the server's proxy's
default server does work but it's a subtle trap. Indeed, no check is made
on srv->proxy to be valid, and this only works because the compiler is
comparing pointer offsets. During the boot, it's common to have NULL here
in srv->proxy and of course in this case srv does not match that value
which is NULL plus epsilon. But when trying to turn defsrv to a dynamic
pointer instead, then the compiler is forced to dereference this NULL
srv->proxy and dies during init.

Let's always add the null check for srv->proxy before the test to avoid
this situation.

No backport is needed since the problem cannot happen yet.