fixes a critical codegen bug triggered by .async

tests/ccgbugs/twrong_string_asgn.nim

@@ -0,0 +1,19 @@
+discard """
+  output: "adf"
+"""
+
+import asyncdispatch
+const
+  test = ["adf"]
+
+proc foo() {.async.} =
+  for i in test:
+    echo(i)
+
+var finished = false
+let x = foo()
+x.callback =
+  proc () =
+    finished = true
+
+while not finished: discard