box: fix NULL pointer dereference in error_unpack_unsafe

If MP_ERROR map contains two MP_ERROR_STACK keys, then the second call to
`error_set_prev(effect, cur)' will crash, because `effect' is NULL, but
`err == NULL' is false, because it is assigned on the first iteration.
This patch raises an error if more than one MP_ERROR_STACK key is present.

NO_DOC=bugfix

Closes #9136