fixes #25121; [FieldDefect] with iterator-loop (#25130)

fixes #25121

compiler/transf.nim

@@ -259,7 +259,8 @@ proc transformBlock(c: PTransf, n: PNode): PNode =
   var labl: PSym
   if c.inlining > 0:
     labl = newLabel(c, n[0])
-    c.transCon.mapping[n[0].sym.itemId] = newSymNode(labl)
+    if n[0].kind != nkEmpty:
+      c.transCon.mapping[n[0].sym.itemId] = newSymNode(labl)
   else:
     labl =
       if n[0].kind != nkEmpty:

tests/iter/titer_issues.nim

@@ -412,3 +412,15 @@ block: # bug #24033
     collections.add (id, str, $num)
 
   doAssert collections[1] == (1, "foo", "3.14")
+
+
+block: # bug #25121
+  iterator k(): int =
+    when nimvm:
+      yield 0
+    else:
+      yield 0
+
+  for _ in k():
+    (proc() = (; let _ = block: 0))()
+