i2v_GENERAL_NAMES call i2v_GENERAL_NAME repeatedly as required. Each
time i2v_GENERAL_NAME gets called it allocates adds data to the passed in
stack and then returns a pointer to the stack, or NULL on failure. If
the passed in stack is itself NULL then it allocates one.

i2v_GENERAL_NAMES was not correctly handling the case where a NULL gets
returned from i2v_GENERAL_NAME. If a stack had already been allocated then
it just leaked it.
Reviewed-by: NDmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: NViktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10300)

(cherry picked from commit 45b244620a74248b46ebe1c85e86437b9641447a)