Keybindings: model reference leak

It looks like the reference created from https://github.com/microsoft/vscode/blob/403be36aebd235bbf00b36b585969a20bc954a6f/src/vs/workbench/services/keybinding/common/keybindingEditing.ts#L226 is potentially not being disposed in error cases. You must always dispose it otherwise a model will hang around forever.