Don't crash if /oem/os-release.d/product_id does not exist.

Now that we have symlinks in os-release.d, it might fail if the file
it points to does not exist, this is not a FATAL error.

Test: update_engine no longer crash when /oem/os-release.d/product_id does not exist
Bug: 32555385
Change-Id: I6c0bc9fa4fdf28ce2e8aef7070e0def7704bd073
(cherry picked from commit 65bf5631440fb3d6887ce3bf5a7d05f31f714421)
1 file changed