fix #19193 (#19195) [backport:1.2]

(cherry picked from commit cd592ed)

compiler/cgen.nim

@@ -439,8 +439,14 @@ proc constructLoc(p: BProc, loc: var TLoc, isTemp = false) =
   if optSeqDestructors in p.config.globalOptions and skipTypes(typ, abstractInst + {tyStatic}).kind in {tyString, tySequence}:
     linefmt(p, cpsStmts, "$1.len = 0; $1.p = NIM_NIL;$n", [rdLoc(loc)])
   elif not isComplexValueType(typ):
-    linefmt(p, cpsStmts, "$1 = ($2)0;$n", [rdLoc(loc),
-      getTypeDesc(p.module, typ, mapTypeChooser(loc))])
+    if containsGarbageCollectedRef(loc.t):
+      var nilLoc: TLoc
+      initLoc(nilLoc, locTemp, loc.lode, OnStack)
+      nilLoc.r = rope("NIM_NIL")
+      genRefAssign(p, loc, nilLoc)
+    else:
+      linefmt(p, cpsStmts, "$1 = ($2)0;$n", [rdLoc(loc),
+        getTypeDesc(p.module, typ, mapTypeChooser(loc))])
   else:
     if not isTemp or containsGarbageCollectedRef(loc.t):
       # don't use nimZeroMem for temporary values for performance if we can