Get rid of g_hash_table_lookup asserts when loading an external host file

Only parse the host file syntax without trying to store the names in the hash tables (it will be done later in host_name_lookup_init())

Change-Id: I2b8c7b29220e6413c1b5c0a0fa238ecb5388c962
Reviewed-on: https://code.wireshark.org/review/4309
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>