Newruntime: Double destruction of seq/string

[jwollen]

The following destroys the inner seq twice with --newruntime, causing unexpected behavior. To have it immediately crash consistently, use -d:useMalloc.
(The Foo is not important. This happens with any non-constant element)

type Foo = object

proc test() =
  var x = @[@[Foo()]]

test()

From the generated code:

T2_.len = 1; T2_.p = (tySequence_YBwSFr9cHnwXey9c6gHCsmuA_Content*) newSeqPayload(1, sizeof(tySequence_9aBObKOIQyNbQpSLHF0GtHw));
T3_.len = 1; T3_.p = (tySequence_9aBObKOIQyNbQpSLHF0GtHw_Content*) newSeqPayload(1, sizeof(tyObject_Foo_pMBAR5nJd0PD9cv5J1Hnd9bA));
...
eqsink__vltVh8v0Jk9bJ9bP86AQ9bCzA(colontmpD_, T3_);
T2_.p->data[0] = colontmpD_;
eqsink__6II9babRqgYpp6uFdPL4n3w(x, T2_);
...
eqdestroy__qK268sotx3d7bmLMEDvQtQ(colontmpD_);
eqdestroy__EKI3QOkat5QRImLwai9aGKw(x);

The inner seq is first deallocated through the temporary, then again as part of the outer seq.

Related (but not fixing the issue): The generated destructors don't nil the payload/length of the sequences.

[sinkingsugar]

I would actually suggest to run any --newruntime test with -d:useMalloc btw.
It's the only way to figure memory corruption reliably and sooner.