yhttpd: fix malformed Content-Length crash and unchecked accept

Two residual stability bugs in yhttpd's own sock.cpp (ychat never had
them -- yhttpd's read_http request parser is structurally different
from ychat's), found while auditing for ychat engine-fix backports:

1. sock::read_http malformed Content-Length crash (a3908e1-class).
   read_http matched the header on the 15-char prefix "Content-Length:"
   (no space required) but then assumed the canonical
   "Content-Length: <value>" form and substr'd from index 16. Two
   reachable crash cases for any unauthenticated client:
     - bare "Content-Length:" (15 chars): substr(16, len-16) had
       pos > size -> std::out_of_range throw -> uncaught -> process crash.
     - "Content-Length:\n" (16 chars, no value): the substring was
       empty so the do/while digit scan read past the buffer (OOB read)
       until a stray '\n' in adjacent memory.
   Now guarded: require the space separator + a value before substr,
   and bound the scan to the substring length. Verified in Docker:
   both malformed cases close gracefully, a valid Content-Length: 0
   POST still returns 200, server stays up.

2. sock::start unchecked accept(). The accept() return was used
   unchecked; on failure (fd == -1, e.g. EMFILE/ENFILE under fd
   exhaustion, EINTR) FD_SET(-1, &active_fd_set) is UB (bit-op on a
   negative index) and the later _create_container(-1) would
   read/write fd -1 (EBADF). Now bails with ACCPERR and continues on
   any accept error (the accept-bail half of ychat's 1c36abe, which
   the original yhttpd port only carried the size_t->socklen_t init of).

yhttpd is not deployed to the cluster (no Helm chart/ArgoCD app); this
is a build-and-verify-in-Docker project, so no deploy step. ycurses
shares no socket/template engine with ychat (it is a standalone curses
library demo) so nothing applies there.

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