i#1073: Initialize prop_idx elements to -1 in handle_execve()

This fixes a UMR regression introduced in r1661.

Fixes issue 1073.

Review URL: https://codereview.appspot.com/7303073

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1889 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6