Fix infinite recursion when await is in except body.

tests/async/tasynctry2.nim

@@ -0,0 +1,16 @@
+discard """
+  file: "tasynctry2.nim"
+  errormsg: "\'yield\' cannot be used within \'try\' in a non-inlined iterator"
+  line: 15
+"""
+import asyncdispatch
+
+proc foo(): Future[bool] {.async.} = discard
+
+proc test5(): Future[int] {.async.} =
+  try:
+    discard await foo()
+    raise newException(ValueError, "Test5")
+  except:
+    discard await foo()
+    result = 0