BUG/MINOR: ssl-sock: do not return an uninitialized pointer in ckch_inst_sni_ctx_to_s...

BUG/MINOR: ssl-sock: do not return an uninitialized pointer in ckch_inst_sni_ctx_to_sni_filters

There's a build error reported here:
   https://github.com/haproxy/haproxy/commit/c9c6cdbf9c0e61ee88d68960a3220a4fcbf912cd/checks?check_suite_id=501365673

It's just caused by an inconditional assignment of tmp_filter to
*sni_filter without having been initialized, though it's harmless because
this return pointer is not used when fcount is NULL, which is the only
case where this happens.

No backport is needed as this was brought today by commit 38df1c8006
("MINOR: ssl/cli: support crt-list filters").