Since values for local variables are referenced by offset within a Frame
(not by identifier name), and event/hook handler bodies share a common
Frame, the value offsets for local variables in different handlers may
overlap. This meant locals in a handler without an initialization may
actually end up referring to the value of a previous handler's local
that has the same Frame offset. When executing the body, that can
possibly result in a type-conflict error or give give unexpected
results instead of a "use of uninitialized value" error.
This patch makes it so uninitialized locals do always refer to a null
value before executing the body of a event/hook handler, so that using
them without assigning a value within the body will connsistently give
a "use of uninitialized value" error.
Addresses #932.
