net: tcp: Fix TCP context cleanup during async handshake

In case socket is closed during an async TCP handshake, the TCP context
should be closed immediately, otherwise the connection could be
established after the socket was closed, causing TCP context leak.
To avoid race between socket close and resend timer (i.e. socket being
closed at the same time as the retransmission limit is reached), add
extra state checks before attempting to close the TCP context.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit 8588b08808e28ee8d6d64398a6f482657c0d32af)