cmd: undefined return value of do_extension_apply()

If 'extension apply all' is executed and no extension is found, the return
value of do_extension_apply() is undefined. Return CMD_RET_FAILURE in this
case.

Fixes: 2f84e9cf ("cmd: add support for a new "extension" command")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>