style: Avoid crashing when calling insertRule("@import ...") on a detached sheet.
style: Avoid crashing when calling insertRule("@import ...") on a detached sheet.

This should unblock the fuzzers for now, though it's not the ideal solution.

It's the only reasonably easy solution to unblock them though, I think.

We should probably always keep track of the document a stylesheet was associated
with. We'll need that for constructible stylesheets anyway.

That requires some though on how to get the cycle-collection and such right,
though, and I wouldn't be able to write or land that ASAP.

Differential Revision: https://phabricator.services.mozilla.com/D23584

nnethercote/servo,saneyuki/servo,nnethercote/servo,splav/servo,notriddle/servo,pyfisch/servo,notriddle/servo,saneyuki/servo,paulrouget/servo,DominoTree/servo,paulrouget/servo,emilio/servo,larsbergstrom/servo,saneyuki/servo,emilio/servo,paulrouget/servo,emilio/servo,emilio/servo,pyfisch/servo,KiChjang/servo,pyfisch/servo,saneyuki/servo,larsbergstrom/servo,splav/servo,larsbergstrom/servo,pyfisch/servo,nnethercote/servo,KiChjang/servo,splav/servo,splav/servo,DominoTree/servo,paulrouget/servo,paulrouget/servo,emilio/servo,KiChjang/servo,paulrouget/servo,emilio/servo,larsbergstrom/servo,splav/servo,emilio/servo,splav/servo,DominoTree/servo,nnethercote/servo,DominoTree/servo,nnethercote/servo,DominoTree/servo,larsbergstrom/servo,emilio/servo,paulrouget/servo,saneyuki/servo,larsbergstrom/servo,saneyuki/servo,saneyuki/servo,notriddle/servo,KiChjang/servo,notriddle/servo,nnethercote/servo,splav/servo,larsbergstrom/servo,DominoTree/servo,larsbergstrom/servo,paulrouget/servo,pyfisch/servo,KiChjang/servo,DominoTree/servo,splav/servo,notriddle/servo,pyfisch/servo,nnethercote/servo,KiChjang/servo,KiChjang/servo,notriddle/servo,splav/servo,pyfisch/servo,emilio/servo,notriddle/servo,pyfisch/servo,emilio/servo,DominoTree/servo,larsbergstrom/servo,KiChjang/servo,nnethercote/servo,paulrouget/servo,DominoTree/servo,notriddle/servo,saneyuki/servo,notriddle/servo,saneyuki/servo,pyfisch/servo,KiChjang/servo,KiChjang/servo,splav/servo,nnethercote/servo,DominoTree/servo,pyfisch/servo,nnethercote/servo,paulrouget/servo,larsbergstrom/servo,notriddle/servo,saneyuki/servo