[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