mime/multipart.Reader.ReadForm allows specifying the maximum amount
of memory that will be consumed by the form. While this limit is
correctly applied to the parsed form data structure, it was not
being applied to individual header lines in a form.

For example, when presented with a form containing a header line
that never ends, ReadForm will continue to read the line until it
runs out of memory.

Limit the amount of data consumed when reading a header.

Fixes CVE-2023-45290
Fixes #65389
For #65383

Change-Id: I7f9264d25752009e95f6b2c80e3d76aaf321d658
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/2134435


Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/2173776


Reviewed-by: Carlos Amedee <amedee@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/569240


Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>