Core: tfm_spm_get_msg_from_handle to check handle

Add extra checks to tfm_spm_get_msg_from_handle to validate message
handler:
 - Check the handler is allocated from the pool
 - Use magic to signal that a handler is active

Change-Id: Ic7b13cc1d61ae48e6112864bb2a1ce2059247e65
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>