Dereferences of NULL after checks

[partimScurra]

Describe the issue
A static analysis tool highlighted possible dereferences at following functions:

/keepalived/vrrp/vrrp.c, open_sockpool_socket:

1. sock->mcast_daddr is used at 2800 after being checked at 2793;
2. sock->ifp is used at 2794 after being checked before the open_sockpool_socket call at vrrp_scheduler.c:555

/lib/vector.c, strvec_slot:
strvec is passed into null_strvec_handler after being checked. The null_strvec function, that is used as null_strvec_handler, doesn't have additional check for strvec.

To Reproduce
Is a code-related issue.

Expected behavior
The NULL checks imply a possibility of NULL being accessed later in the code. Perhaps existing checks need to be revised to prevent such possibility.

Keepalived version
2.3.4