empty block on s-for causes compiler error

[jpolitz]

for map(): end

causes

Expected to get Srcloc because of the annotation at file:///home/travis/build/brownplt/code.pyret.org/node_modules/pyret-lang/src/arr/compiler/compile-structs.arr:434:26-434:31 at (file:///home/travis/build/brownplt/code.pyret.org/node_modules/pyret-lang/src/arr/compiler/compile-structs.arr:434:26-434:31) but got:

nothing
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It seems that parent-block-loc is not reliably updated in well-formed.arr

[blerner]

I'll take care of this