console: Fix warning from clang (and potential crash)

ccc-analyzer reports this warning:

console.c:1090:29: warning: Dereference of null pointer
        if (active_console->cursor_timer) {
                            ^

Function console_select allows active_console to be NULL,
but would crash when accessing cursor_timer. Fix this.

Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
