[9.18] Parse statschannel Content-Length: more carefully

A negative or excessively large Content-Length could cause a crash by making INSIST(httpd->consume != 0) fail.

(cherry picked from commit 26e10e8f)

Closes #4125 (closed)

Backport of !8018 (merged) and !8216 (merged)

Edited by Ondřej Surý

Merge request reports