Don't assume we have interface information.

Not all capture file formats provide interface information.  Check
whether we have it, and don't fail if we don't have any.

(Note that the current version of the pcapng specification in the Git
repository does not require that there be at least one IDB in a file,
only that there must be an IDB for each interface to which other blocks
in a file refers.)

Change-Id: Ia796f639ed3a8cb21e1dbe9b2bc6b4b66c7be479
Reviewed-on: https://code.wireshark.org/review/10220
Reviewed-by: Guy Harris <guy@alum.mit.edu>

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