@model macro breaks on conditional branches with return values

[bgroenks96]

As discussed with @torfjelde on Slack, here is a minimal example:

@model function demo()
    x ~ Normal(0,1)
    if true
       return x+1
    else
       return x-1
    end
end

@model function superdemo()
    @submodel x = demo()
end

which results in:

ERROR: BoundsError: attempt to access Float64 at index [2]
Stacktrace:
  [1] indexed_iterate(I::Float64, i::Int64, state::Nothing)
    @ Base ./tuple.jl:97
  [2] macro expansion
    @ ~/.julia/packages/DynamicPPL/oJMmE/src/submodel_macro.jl:228 [inlined]
  [3] macro expansion
    @ ~/.julia/packages/DynamicPPL/oJMmE/src/compiler.jl:487 [inlined]
...

In other cases, depending on what is returned, the error may be different.

[torfjelde]

Just to it's clear, this does not require usage of @submodel:

julia> @model function demo()
           if true
               return nothing
           else
               return 0
           end
       end
demo (generic function with 2 methods)

julia> demo()()
ERROR: MethodError: no method matching iterate(::Nothing)

Closest candidates are:
  iterate(::Union{LinRange, StepRangeLen})
   @ Base range.jl:880
  iterate(::Union{LinRange, StepRangeLen}, ::Integer)
   @ Base range.jl:880
  iterate(::T) where T<:Union{Base.KeySet{<:Any, <:Dict}, Base.ValueIterator{<:Dict}}
   @ Base dict.jl:698
  ...

Stacktrace:
 [1] first(itr::Nothing)
   @ Base ./abstractarray.jl:465
 [2] (::Model{typeof(demo), (), (), (), Tuple{}, Tuple{}, DefaultContext})()
   @ DynamicPPL /drive-2/Projects/public/DynamicPPL.jl/src/model.jl:860
 [3] top-level scope
   @ REPL[20]:1

[torfjelde]

This should be fixed in #512 :)