BUILD: ssl: avoid a wrong null deref warning in ssl_sock_handshake

BUILD: ssl: avoid a wrong null deref warning in ssl_sock_handshake

When disabling traces, "conn" isn't used between ctx assignment and its
first usage, and as usual, gcc wrongly believes that a null check in a
shared function implies the checked argument may be NULL where it's used,
leading to this warning:

  src/ssl_sock.c: In function 'ssl_sock_handshake.constprop':
  src/ssl_sock.c:6049:7: warning: null pointer dereference [-Wnull-dereference]

Assigning ctx after the conn_ctrl_ready() check is sufficient to shut it
up, so let's do this. It should also result in slightly better code.