server/telnet: Remove exit() call

Change-Id: I8ce99d7d62dfe3cad63cf6bc68f2faf2234e395c
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3224
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/server/telnet_server.c

@@ -573,7 +573,7 @@ static int telnet_input(struct connection *connection)
 				break;
 			default:
 				LOG_ERROR("unknown telnet state");
-				exit(-1);
+				return ERROR_FAIL;
 		}
 
 		bytes_read--;