eCryptfs: decrypt_pki_encrypted_session_key(): remove kfree() redundant null check

smatch analysis:

fs/ecryptfs/keystore.c:1206 decrypt_pki_encrypted_session_key() info:
 redundant null check on msg calling kfree()

Cc: Dustin Kirkland <dustin.kirkland@gazzang.com>
Cc: ecryptfs@vger.kernel.org
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>