handle empty bus_num

[adzierzanowski]

When $bus_num is empty (e.g. in case of 05ac:8102 Apple Inc. Apple T2 Bus), the script throws the following error:

/usr/local/bin/lsusb: line 89: 16#: invalid integer constant (error token is "16#")

because it is not possible to treat an empty variable as a hexadecimal and convert it into an integer. My fix handles the situation by printing --- when the bus number is not provided.

[reitermarkus]

@jlhonora, can this be merged?

[robertlipe]

This is the same problem fixed - in a different way - in #18 also, so if you merge this one you can get a two-fer.
#18 was a confirmed fix for the version I installed via homebrew.