releasetools: Fix a bug in ReviseStashSize().

We check the needed stash size in ReviseStashSize(), and may not
generate a stash command if it would exceed the max allowed size. This
CL fixes a bug when skipping a stash operation: we shouldn't update the
'stashes' map if a stash command won't be generated.

Bug: 35313668
Test: Successfully generate the package that was failing due to the bug.
Change-Id: If0a3a5fadda9b4a4edad66a2a5826b5f978400ae