sbin/dhclient: fix a memory leak in parse_client_lease_statement(..)

The memory stored by lease would have previously been leaked if an
unterminated lease declaration was found in an early-return code path.

MFC after: 1 week
Reported by: clang static analyzer, Coverity
CID: 1007114
Submitted by: Tom Rix <trix@juniper.net>
Sponsored by: Dell EMC Isilon; Juniper, Inc
Differential Revision: D9992