vulndb/internal/audit: don't compute trace twice
vulndb/internal/audit: don't compute trace twice

Avoid generating a import path trace a second time
just to take its length.

Change-Id: I9085c66cbe814ce127c531fed76f996d5fc36686
Reviewed-on: https://go-review.googlesource.com/c/exp/+/390554
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Zvonimir Pavlinovic <490af24f3def1258d2f7e9b10b20e341f6c68320@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/exp,golang/exp,golang/exp