net: don't allocate dialer closure if we won't use it

When we use dialMulti we also allocate dialSingle closure for no reason.

Change-Id: I074282a9d6e2c2a1063ab311a1b95e10fe65219f
Reviewed-on: https://go-review.googlesource.com/4119


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>