fixes #12747 [backport] (#13651)

* fixes #12747
* fix tests
* improve code style

Co-authored-by: cooldome <ariabushenko@bk.ru>

tests/destructor/tgcdestructors.nim

@@ -10,7 +10,7 @@ a: @[4, 2, 3]
 0
 30
 true
-(allocCount: 41, deallocCount: 41)'''
+(allocCount: 40, deallocCount: 40)'''
 """
 
 include system / ansi_c

tests/destructor/twidgets_unown.nim

@@ -2,7 +2,7 @@ discard """
   cmd: '''nim c -d:nimAllocStats --newruntime $file'''
   output: '''button
 clicked!
-(allocCount: 9, deallocCount: 9)'''
+(allocCount: 7, deallocCount: 7)'''
 """
 
 import system / ansi_c

tests/iter/titer11.nim

@@ -26,3 +26,14 @@ var output = represent(s)
 
 for item in output():
   echo item
+
+
+#------------------------------------------------------------------------------
+# Issue #12747
+
+type
+  ABC = ref object
+      arr: array[0x40000, pointer]
+let a = ABC()
+for a in a.arr:
+    assert a == nil