Ensure only initialised values are serialised.

[samhocevar]

For performance reasons Lua does not initialise memory until it knows
the values will actually be used. However Eris serializes some of them
without knowing whether they have been initialised, causing harmless
(but verbose) Valgrind memory check errors.

[samhocevar]

While debugging Eris I have come across a few Valgrind warnings about uninitialised call info memory being serialised. This is harmless but does pollute error reporting when investigating issues, so I thought I’d suggest the attached changes for review.