Prevent use-after-free bugs in object variants. Fixes bug #20305 (#20300) [backport]

prevent use-after-free bugs in cased objects

the bug happens specifically when deleting
an item in a seq. The item taking it's place
might not have the same case fields. Then =sink(x[i], move x[xl])
might leave the deleted fields still in memory!
If the new item switches branches again, you get a use-after-free bug.

compiler/liftdestructors.nim

@@ -165,9 +165,12 @@ proc fillBodyObj(c: var TLiftCtx; n, body, x, y: PNode; enforceDefaultOp: bool)
       # the value needs to be destroyed before we assign the selector
       # or the value is lost
       let prevKind = c.kind
+      let prevAddMemReset = c.addMemReset
       c.kind = attachedDestructor
+      c.addMemReset = true
       fillBodyObj(c, n, body, x, y, enforceDefaultOp = false)
       c.kind = prevKind
+      c.addMemReset = prevAddMemReset
       localEnforceDefaultOp = true
 
     if c.kind != attachedDestructor: