encoding/json: limit max nesting depth

[sthagen]

Limit the maximum nesting depth when parsing to protect against stack
overflow, permitted by https://tools.ietf.org/html/rfc7159#section-9

A nesting depth limit of 10,000 was chosen to be a conservative
balance between avoiding stack overflow and avoiding impacting
legitimate JSON documents.

10,000 is less than 1% of the experimental stack depth limit
with the default stack size:

• On 64-bit systems, the default stack limit is 1GB,
  which allows ~2,800,000 frames of recursive parsing
• On 32-bit systems, the default stack limit is 250MB,
  which allows ~1,100,000 frames of recursive parsing

Fixes golang#31789

Change-Id: I4f5a90e89dcb4ab1a957ad9d02e1fa0efafaccf6
Reviewed-on: https://go-review.googlesource.com/c/go/+/199837
Run-TryBot: Daniel Martí mvdan@mvdan.cc
TryBot-Result: Gobot Gobot gobot@golang.org
Reviewed-by: Daniel Martí mvdan@mvdan.cc

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