v4.0.x: libnbc uninitialized variable

[jsquyres]

While working on #8258, I found one more warning on the v4.0.x branch -- it happens many times:

./nbc_internal.h:538:60: warning: variable 'size' is uninitialized when used here [-Wuninitialized]
    res = ompi_datatype_unpack_external("external32", src, size, &pos, tgt, srccount, srctype);
                                                          ^~~~
./nbc_internal.h:515:16: note: initialize the variable 'size' to silence this warning
  MPI_Aint size, pos;
               ^

This warning does, indeed, show that there is a legit code path where size can be uninitialized. This should be fixed.

However, the code in nbc_internal.h appears to be different on v4.1.x. I'm not sure where the divergence happened, and I don't have time to track it down at the moment (there could be just another commit to cherry pick from v4.1.x->v4.0.x, or if that is too large, a smaller fix that could locally be applied directly to v4.0.x).

I'm therefore filing this issue so that someone fixes it before v4.0.6 is released. I'm tentatively marking this a "blocker" so that the RMs are definitely aware of it. They can choose to downgrade it if they wish.

[devreal]

It seems that d9d84d5 was not backported to v4.0.x, which fixed this issue.

[jsquyres]

Same seems very straightforward. Can someone cherry-pick it?

[jsquyres]

Actually, I just added d9d84d5 to #8258. Straightforward.