BUILD: ssl-sock: Silent error about NULL deref in ssl_sock_bind_verifycbk()

BUILD: ssl-sock: Silent error about NULL deref in ssl_sock_bind_verifycbk()

In ssl_sock_bind_verifycbk(), when compiled without QUIC support, the
compiler may report an error during compilation about a possible NULL
dereference:

src/ssl_sock.c: In function ‘ssl_sock_bind_verifycbk’:
src/ssl_sock.c:1738:12: error: potential null pointer dereference [-Werror=null-dereference]
 1738 |         ctx->xprt_st |= SSL_SOCK_ST_FL_VERIFY_DONE;
      |         ~~~^~~~~~~~~

A BUG_ON() was addeded because it must never happen. But when compiled
without DEBUG_STRICT, there is nothing to help the compiler. Thus
ALREADY_CHECKED() macro is used. The ssl-sock context and the bind config
are concerned.

This patch must be backported to 2.6.