libdexfile: Fix inconsequential resource leak in test

We make sure to close 'output_fd' in an error case.  This is
inconsequential since the test terminates almost immediately
after this.  But fixing this helps static analysis find other
leaks in our code.

Test: TreeHugger
Change-Id: Id3392c366d575dc93f3e1ad802e436b44dda0870