Fix #1170.

tests/async/tasyncdiscard.nim

@@ -0,0 +1,39 @@
+discard """
+  output: '''
+1
+2
+3
+4
+1
+2
+1
+6
+'''
+"""
+import asyncio, asyncdispatch, asyncnet
+
+proc main {.async.} =
+  proc f: PFuture[int] {.async.} =
+    discard
+    echo 1
+    discard
+    result = 2
+    discard
+
+  let x = await f()
+  echo x
+  echo 3
+
+  proc g: PFuture[int] {.async.} =
+    discard
+    echo 4
+    discard
+    result = 6
+    discard
+    echo await f()
+    discard await f()
+
+  discard await g()
+  echo 6
+
+main()