encoding/gob: don't drop entries from large map

CL 781684 had a bug: if a large map was broken into chunks,
it would only decode a single chunk.

For #47653
For #79539
Fixes #79756

Change-Id: I42451932e3972cd23bf8c019dc9e36683c6f4550
Reviewed-on: https://go-review.googlesource.com/c/go/+/785421
Reviewed-by: Neal Patel <neal@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: David Chase <drchase@google.com>