Correct execute() to check 0 in check_exit_code

This patch fixes: if a process exits with returncode 0, the function
always returns without raising an exception, regardless of whether
its check_exit_code includes 0 or not.

Change-Id: Ibdf7599dd91949153629083e4c5a47d263d6d002
Merge "Correct execute() to check 0 in check_exit_code"