:bug: remove UB
:bug: remove UB

The UB arose due to `self.context.para` being `MaybeUninit::zeroed`,
which is an invalid value for an `*mut dyn T`, as fat pointers' vtable
pointer cannot be null.

Which means that during the phase on the left-hand side of the `=`, an
invalid object was created, which lead to UB.

This, in turn, manifested in
https://github.com/carllerche/loom/issues/50

Xudong-Huang/generator-rs