This patch improves the resilience of the guess_icon function by sanitizing mac, vendor, and name fields to avoid crashes caused by unexpected data types (e.g., numeric hostnames).
Specifically:
mac is now cast to a string before being uppercased, with a newly added fallback to "00:00:00:00:00:00" if empty or invalid.
vendor is sanitized to a string before lowercasing, still defaulting to "unknown".
name is cast to a string before lowercasing, still falling back to "(unknown)" when empty.
This change not only resolves the error caused by numeric-only hostnames (which triggered an AttributeError due to calling .lower() on an int), but also proactively prevents similar crashes from malformed or unexpected input in the future.
References: Fixes issue #1088 and also let's me sleep a little easier tonight.
