libadf: adf_test: fix crash on adf.devices failure

If devs is uninitialized and adf_devices() fails, we'll end up passing
the uninitialized pointer to free().

Test: /data/nativetest64/adf-unit-tests/adf-unit-tests (on Nexus 9 w/o root)
Change-Id: Ifc6038c1da14d32ee564675bac54fc7df2623c1d
Signed-off-by: Greg Hackmann <ghackmann@google.com>