net/http: correct Content-Length parsing for js/wasm

The Content-Length was incorrectly set to 0 for ill-formed and invalid
values. In these cases, return an error.

If the Content-Length header was omitted, it was incorrectly set to 0.
In this case, set the Content-Length value to -1.

Fixes #49108

Change-Id: I24fe9a31ed5b6ddb53f2b2bd10f2c84e428823e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/358134


Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Trust: David Crawshaw <crawshaw@golang.org>