The failure of the housing will not result in the derailleur falling into the wheel, simply because the range of the derailleur is determined by the limit screws, not by the derailleur cable. If your cable housing is the constant-length type (i.e., correct for derailleurs but may be deadly for brakes), just wrapping the damaged part with tape will probably be OK for a while. Don't use electrical tape as that may be too stretchy; use something like fiberglass packing tape or another tape that is dimensionally stable.

May I suggest just visiting your local bike shop and asking for an old length of housing? Bike shops usually have bits of used housing in the trash that are not new but may be better than your damaged housing.