Make closured work in blocks that don't closure vars but an outer context has closured vars

spec/compiler/codegen/closure_spec.cr

@@ -135,6 +135,20 @@ describe "Code gen: closure" do
       ").to_i.should eq(9)
   end
 
+  it "codegens closure with nested context without new closured vars" do
+    run("
+      def foo
+        yield
+      end
+
+      a = 1
+      f = foo do
+        -> { a }
+      end
+      f.call
+      ").to_i.should eq(1)
+  end
+
   # pending "transforms block to fun literal" do
   #   run("
   #     def foo(&block : Int32 ->)

src/compiler/crystal/codegen.cr

@@ -2635,15 +2635,19 @@ module Crystal
     end
 
     def malloc_closure(closure_vars, current_context, parent_context = nil)
-      if closure_vars
-        closure_type = @llvm_typer.closure_context_type(closure_vars, parent_context.try &.closure_type)
+      parent_closure_type = parent_context.try &.closure_type
+
+      if closure_vars || parent_closure_type
+        closure_vars ||= [] of MetaVar
+
+        closure_type = @llvm_typer.closure_context_type(closure_vars, parent_closure_type)
         closure_ptr = malloc closure_type
         closure_vars.each_with_index do |var, i|
           current_context.vars[var.name] = LLVMVar.new(gep(closure_ptr, 0, i, var.name), var.type)
         end
 
-        if parent_context && parent_context.closure_type
-          store parent_context.closure_ptr.not_nil!, gep(closure_ptr, 0, closure_vars.length, "parent")
+        if parent_closure_type
+          store parent_context.not_nil!.closure_ptr.not_nil!, gep(closure_ptr, 0, closure_vars.length, "parent")
         end
       end