builtin/checkout.c: don't leak memory in check_tracking_name

remote_find_tracking() populates the query struct with an allocated
string in the dst member.  So, we do not need to xstrdup() the string,
since we can transfer ownership from the query struct (which will go
out of scope at the end of this function) to our callback struct, but
we must free the string if it will not be used so we will not leak
memory.

Let's do so.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>