pkcs11-spy.c check return code

The po->C_GetInterface is passed the callers ppInterface where
*ppInterface may not be valid.

if the po->C_GetInterface may not update the *ppInterface and return
an error. In this case  spy_interface_function_list should not be called,
as it assumes the *ppInterface has been modified.

Found debugging FireFox version 121 where FireFox passes a ppInterface
where *ppInterface is not a valid pointer, causing a segfault in
spy_interface_function_list.

FireFox calls C_GetInterface twice with flags = CKF_INTERFACE_FORK_SAFE
twice then on third time requests with flag = 0  where po->GetInterface
can support and it updates the *ppInterface  with valid data.

See OpenSC#2987

 On branch pkcs11-spy-segfault
 Changes to be committed:
	modified:   pkcs11-spy.c

src/pkcs11/pkcs11-spy.c

@@ -1672,7 +1672,7 @@ C_GetInterface(CK_UTF8CHAR_PTR pInterfaceName, CK_VERSION_PTR pVersion,
 		(flags & CKF_INTERFACE_FORK_SAFE ? "CKF_INTERFACE_FORK_SAFE" : ""));
 	if (po->version.major >= 3) {
 		rv = po->C_GetInterface(pInterfaceName, pVersion, ppInterface, flags);
-		if (ppInterface != NULL) {
+		if (rv == CKR_OK && ppInterface != NULL) {
 			spy_interface_function_list(*ppInterface);
 		}
 	} else {