[dev.fuzz] internal/fuzz: fix panic when marshaling data

[dev.fuzz] internal/fuzz: fix panic when marshaling data

The coordinator needs to marshal data that was provided
via f.Add. However, it was also attempting to marshal data
that was in testdata, which was not needed,
and was causing a panic. This change fixes this.

Fixes golang/go#48228

Change-Id: I1256c5a287b5a09d2f8cca59beb0f0fc06cc3554
Reviewed-on: https://go-review.googlesource.com/c/go/+/348381
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>