Prevent Context.RemoteAddr() from exploding when called from ConnCallback

When calling ctx.RemoteAddr() within ConnCallback one gets a panic due to
unsafe casting within the accessor. I understand it's not a valid scenario
as such, but I accidentally called a logger that expected that checked
for RemoteAddr() in ssh.Context

panic: interface conversion: interface is nil, not net.Addr

Signed-off-by: Alejandro Mery <amery@geeks.cl>

context.go

@@ -140,7 +140,10 @@ func (ctx *sshContext) ServerVersion() string {
 }
 
 func (ctx *sshContext) RemoteAddr() net.Addr {
-	return ctx.Value(ContextKeyRemoteAddr).(net.Addr)
+	if addr, ok := ctx.Value(ContextKeyRemoteAddr).(net.Addr); ok {
+		return addr
+	}
+	return nil
 }
 
 func (ctx *sshContext) LocalAddr() net.Addr {