Empty module crashes interpreter

[KristofferC]

julia> stack = JuliaInterpreter.JuliaStackFrame[];

julia> JuliaInterpreter.interpret!(stack, Main, :(module Foo end))
ERROR: UndefVarError: ret not defined
Stacktrace:
 [1] interpret!(::Array{JuliaStackFrame,1}, ::Module, ::Expr) at C:\Users\Kristoffer\Debugging\JuliaInterpreter\src\JuliaInterpreter.jl:788
 [2] interpret!(::Array{JuliaStackFrame,1}, ::Module, ::Expr) at C:\Users\Kristoffer\Debugging\JuliaInterpreter\src\JuliaInterpreter.jl:769
 [3] top-level scope at none:0

[KristofferC]

Closed by #43