Handle new vendors and changes to vendors differently.

If, when adding a new vendor, we already find a vendor with the given
ID, don't bother adding it to the vendors-by-ID table with the same ID,
and, if and only if the vendor name has changed, remove it from the
vendors-by-name table, free the old name, replace it with the new name,
and, add it back to the vendors-by-name table with the new name.

Bug: 10666
Change-Id: I43ebcb57c742563157c71b01ebc0b35797a408e1
Reviewed-on: https://code.wireshark.org/review/5265
Reviewed-by: Guy Harris <guy@alum.mit.edu>