stack.py: Fix script execution in fallback .so code path

The last change to this script had issues in cases where an APK can't be
found or isn't specified.

TBR'ing Andrew to get this in quickly, as it's trivial fixes but
potentially disruptive to bots.

TBR=agrieve@chromium.org

Bug: 1015159
Change-Id: Iddd45829abdd11478cd3c9aacb4176ea72e49258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914826
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Auto-Submit: Christopher Grant <cjgrant@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#715130}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1d768300f373080db3a2a91b26d17e814e62df79