This commit fixes an issue where when looking up the stack map for a pc
within a function we might end up reading the *previous* function's
stack maps. This then later caused asserts to trip because we started
interpreting random data as a `VMExternRef` when it wasn't. The fix was
to add `None` markers for "this range has no stack map" in the function
ranges map.
Closes#2386
068340d30f