erts: Fix possible heap corruption getting atomics

Due to comparison as a signed integer, when getting an unsigned atomic
in the range 2^63-1..2^64-1 (when the most significant bit was set), the
heap could get corrupted when the integer was retrieved: hsz would get
set to zero, but the code proceeded to build a bignum.

Steps to reproduce (at least on x86_64):

   $ erl
   1> A = atomics:new(1,[{signed,false}]).
   2> atomics:put(A,1,18446744073709551615).
   3> atomics:get(A,1).

At the last step, the shell would print some garbage and hang.

0 files changed, 0 insertions, 0 deletions