BUG/MAJOR: dns: fix null pointer dereference in snr_update_srv_status

BUG/MAJOR: dns: fix null pointer dereference in snr_update_srv_status

Since commit 13a9232eb ("MEDIUM: dns: use Additional records from SRV
responses"), a struct server can have a NULL dns_requester->resolution,
when SRV records are used and DNS answers contain an Additional section.

This is a problem when we call snr_update_srv_status() because it does
not check that resolution is NULL, and dereferences it. This patch
simply adds a test for resolution being NULL. When that happens, it means
we are using SRV records with Additional records, and an entry was removed.

This should fix issue #775.
This should be backported to 2.2.