Avoid crash when checking property compatibility without link info

Teach the compatibility check added by commit 042ecf04 (Add API to
calculate link-interface-dependent bool properties or error, 2013-01-06)
to return early if no link information is available.  This avoids
crashing in a case that should fail with an error message.
Merge topic 'property-link-depends-no-crash' into next

ab079ee Avoid crash when checking property compatibility without link info