BUG/MEDIUM: resolvers: Test for empty tree when getting a record from DNS answer

BUG/MEDIUM: resolvers: Test for empty tree when getting a record from DNS answer

In the previous fix 5d1d93fad ("BUG/MEDIUM: resolvers: Properly handle empty
tree when getting a record from the DNS answer"), I missed the fact the
answer tree can be empty.

So, to avoid crashes, when the answer tree is empty, we immediately exit
from resolv_get_ip_from_response() function with RSLV_UPD_NO_IP_FOUND. In
addition, when a record is removed from the tree, we take care to reset the
next node saved if necessary.

This patch must be backported with the commit above.