Initialize PSA key slot as invalid in ssl_client2.c

Otherwise, if `mbedtls_psa_get_free_key_slot()` fails to find a fresh
key slot, the slot value will be undefined, and the call to
`psa_destroy_key()` at the end of `main()` is undefined behavior.

programs/ssl/ssl_client2.c

@@ -561,7 +561,7 @@ int main( int argc, char *argv[] )
     const char *pers = "ssl_client2";
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
-    psa_key_slot_t slot;
+    psa_key_slot_t slot = 0;
     psa_algorithm_t alg = 0;
     psa_key_policy_t policy;
     psa_status_t status;