Don't allocate the interfaces array until we've read the SHB.

Nothing from the SHB gets stored in the interfaces array - it's filled
in from IDBs - so it doesn't need to exist when we read the first SHB,
and thus doesn't need to be freed if the attempt to read the SHB gets an
error or a "this isn't a pcap-ng file" indication.

Update a comment while we're at it.

Change-Id: Ie67edb52dcf13c974607e95e290661bf48be68ae
Reviewed-on: https://code.wireshark.org/review/4711
Reviewed-by: Guy Harris <guy@alum.mit.edu>