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Fix pam_get_data stack overwrite by saving a heap pointer instead

The previous code was using a trick of saving the actual retval value
as the "pointer". The problem with that was when pam_get_data copied
it out it treated it as a void* which is 8 byte on 64 bit operating
system which meant it copied 8 byte to a 4 byte location and overwrote
the stack with 4 bytes.

The fix is using a heap pointer instead, influenced by the official
code in https://github.com/linux-pam/linux-pam/blob/master/modules/pam_unix/pam_unix_auth.c

With feedback from pedro martelletto, thanks.

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