fix #9872: setLen now works properly at CT [backport]

(cherry picked from commit f838b1e6c1)

tests/vm/tsetlen.nim

@@ -0,0 +1,30 @@
+type Foo = object
+  index: int
+
+block:
+  proc fun[T]() =
+    var foo: T
+    var n = 10
+
+    var foos: seq[T]
+    foos.setLen n
+
+    n.inc
+    foos.setLen n
+
+    for i in 0 ..< n:
+      let temp = foos[i]
+      when T is object:
+        doAssert temp.index == 0
+      when T is ref object:
+        doAssert temp == nil
+      doAssert temp == foo
+
+  static:
+    fun[Foo]()
+    fun[int]()
+    fun[float]()
+    fun[string]()
+    fun[(int, string)]()
+    fun[ref Foo]()
+    fun[seq[int]]()