If any of the TEST_ASSERT()s that are before the call to
mbedtls_pk_warp_as_opaque() failed, when reaching the exit label
psa_destroy_key() would be called with an uninitialized argument.
Found by Clang.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>