fix crash when sorting declarations with no USR
fix crash when sorting declarations with no USR

since usr isn't present in all declarations, we need to fall back on
something.

fixes #661.

agentk/jazzy,PSPDFKit-labs/jazzy,agentk/jazzy,1ec5/jazzy,1ec5/jazzy,realm/jazzy,realm/jazzy,PSPDFKit-labs/jazzy,agentk/jazzy,1ec5/jazzy