BUG/MINOR: server: fix srv_drop() crash on partially init srv

BUG/MINOR: server: fix srv_drop() crash on partially init srv

A recent patch has introduced free operation for QUIC tokens stored in a
server. These values are located in <per_thr> server array.

However, a server instance may be released prior to its full
initialization in case of a failure during "add server" CLI command. The
mentionned patch would cause a srv_drop() crash due to an invalid usage
of NULL <per_thr> member.

Fix this by adding a check on <per_thr> prior to dereference it in
srv_drop().

No need to backport.