Avoid NULL pointer dereference in SMBsendend handler

The "reply_sendend" function wouldn't check whether the connection had
any pending message state. A client sending an out-of-order SMBsendend
message would trigger a NULL pointer dereference.

Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Signed-off-by: Michael Hanselmann <public@hansmi.ch>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>