dashboard: add sanity check
dashboard: add sanity check

Because almost all the BuildConfig methods indirectly call AllScript,
add in a little defense against accident creation of BuildConfig zero
values (of the value type, not nil pointers). This can happen by doing
a lookup in a map[string]dashboard.BuildConfig and then calling a
method on it. We should probably move everything to pointer values of
BuildConfig. There aren't many value types remaining.

This paranoia isn't a result of any bug yet, but rather than the result
of discussion on CL 40477.

Change-Id: Ic273d4de28ac8930b56bc232bf77a9ba39f99fad
Reviewed-on: https://go-review.googlesource.com/40497
Reviewed-by: Herbie Ong <86f376bf42b6daa37c768c730acd318ab6855de4@google.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build