encoding/gob: work around TestFuzzOneByte panic

The index 248 results in the decoder calling reflect.MakeMapWithSize
with a size of 14754407682 - just under 15GB - which ends up in a
runtime out of memory panic after some recent runtime changes on
machines with 8GB of memory.

Until that is fixed in either runtime or gob, skip the troublesome
index.

Updates #24308.

Change-Id: Ia450217271c983e7386ba2f3f88c9ba50aa346f4
Reviewed-on: https://go-review.googlesource.com/99655


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>