Currently, if the number of leading zeros is greater than fits into a
complete limb, mpi_read_buffer() skips them by iterating over them
limb-wise.

Instead of skipping the high order zero limbs within the loop as shown
above, adjust the copying loop's bounds.
Signed-off-by: NNicolai Stange <nicstange@gmail.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>