Fix JS error in ve.ui.ClippableContainer

Sometimes, .clip() would be called when clipping had already been
disabled again, and accessing this.$clippableContainer would cause
an error. This happened to me when typing quickly in MWLinkInspector.

This probably happens when enabling and disabling clipping right after
each other in response to the same event, such that the setTimeout()
for the initial clip is run after clipping has already been disabled.

Change-Id: I0f85ea5f10955188bae51160e1c88eca858ea1bf
Merge "Fix JS error in ve.ui.ClippableContainer"