fixes 'gcsafe'

tests/effects/teffects1.nim

@@ -1,5 +1,5 @@
 discard """
-  line: 1913
+  line: 2136
   file: "system.nim"
   errormsg: "can raise an unlisted exception: ref EIO"
 """

tests/effects/tgcsafe.nim

@@ -0,0 +1,16 @@
+discard """
+  line: 15
+  errormsg: "'mainUnsafe' is not GC-safe"
+"""
+
+proc mymap(x: proc ()) =
+  x()
+
+var
+  myglob: string
+
+proc mainSafe() {.gcsafe.} =
+  mymap(proc () = echo "foo")
+
+proc mainUnsafe() {.gcsafe.} =
+  mymap(proc () = myglob = "bar"; echo "foo", myglob)