Port fix for CVE-2024-24786

[tdeebswihart]

Official description from NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-24786

From the official commit:

encoding/protojson, internal/encoding/json: handle missing object values

In internal/encoding/json, report an error when encountering a }
when we are expecting an object field value. For example, the input
{"":} now correctly results in an error at the closing } token.

In encoding/protojson, check for an unexpected EOF token in
skipJSONValue. This is redundant with the check in internal/encoding/json,
but adds a bit more defense against any other similar bugs that
might exist.

Fixes CVE-2024-24786

Change-Id: I03d52512acb5091c8549e31ca74541d57e56c99d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/569356
TryBot-Bypass: Damien Neil dneil@google.com
Reviewed-by: Roland Shoemaker roland@golang.org
Commit-Queue: Damien Neil dneil@google.com