resolv: free only initialized items from gai pool

pool_max_size denotes total allocated rows in pool but possibly not yet
initialized. it's pool_size that represents number of actually occupied
rows hence use it when freeing pool to avoid freeing random addresses.

Signed-off-by: Jan Palus <jpalus@fastmail.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>