ta: os_test: do not call TEE_CloseTASession() if session is not opened

The Global Platform specification [1] tells us that the session
parameter of TEE_CloseTASession() is:

  An opened session handle

The behaviour is unspecified if the session handle is not opened.

Make sure not to call TEE_CloseTASession() with an invalid session
handle when TEE_OpenTASession() fails by either:

  - doing an early return
  - adding an additional cleanup label

[1] TEE Internal Core API Specification – Public Release v1.3.1,
    §4.9.2 "TEE_CloseTASession"

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>