libgcrypt/mpi: Fix possible unintended sign extension

libgcrypt/mpi: Fix possible unintended sign extension

The array of unsigned char gets promoted to a signed 32-bit int before
it is finally promoted to a size_t. There is the possibility that this
may result in the signed-bit being set for the intermediate signed
32-bit int. We should ensure that the promotion is to the correct type
before we bitwise-OR the values.

Fixes: CID 96697
Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>