internal/fuzz: don't use dirty coverage maps during minimization

When minimizing a value, if the value cannot be minimized (i.e. it is
the final value is the same value as was sent for minimization) return
the initial coverage map, rather than the coverageSnapshot, which is
actually the coverage map for the final minimization step and may not
accurately reflect whether the input actually expands the coverage set
or not.

Updates #48326

Change-Id: I01f0eebe5841e808b6799647d2e5fe3aa45cd2e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/391614


Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>