sbin/dhclient: fix vendor storage leak in parse_option_decl(..)

This ensures the storage isn't leaked when non-NULL and the function
returns early, prior to the free(vendor) later on in the function.

MFC after: 1 week
Reported by: Coverity
CID: 1007111-1007113
Reviewed by: cem
Sponsored by: Dell EMC Isilon
Differential Revision: D9993