Async MySQL extension creating invalid Map in HHVM 4.80

**Describe the bug**
Experiencing a significant rate of segfaults after upgrading to HHVM 4.80

**Standalone code, or other way to reproduce the problem**

This is reproducing for us at a rate of a few times per hour on a host running this code, but we have not found a standalone repro:
```
// $result is an AsyncMysqlQueryResult
$rows = varray[];
foreach ($result->mapRowsTyped() as $row) {
$rows[] = $row->toDArray();
}
return $rows;
```

The PHP stack trace points to the line on which we do `$row->toDArray()`, and the C++ stacks from gdb look like this:
```
#0 raise (sig=) at ../sysdeps/unix/sysv/linux/raise.c:51
#1 0x0000562fddce4dc9 in HPHP::bt_handler (sigin=, info=) at /build/hhvm/hphp/runtime/base/crash-reporter.cpp:270
#2
#3 HPHP::BespokeArray::IterAdvance (ad=0x7f8942faa140, pos=140227510968640) at /build/hhvm/hphp/runtime/base/bespoke-array.cpp:175
#4 0x0000562fddc386e1 in HPHP::ArrayData::iter_begin (this=0x7f8942faa140) at /build/hhvm/hphp/runtime/base/array-data-defs.h:168
#5 HPHP::ArrayIter::setArrayData (ad=0x7f8942faa140, this=0x7f89b9ffb0b0) at /build/hhvm/hphp/runtime/base/array-iterator.h:197
#6 HPHP::ArrayIter::arrInit (this=0x7f89b9ffb0b0, arr=0x7f8942faa140) at /build/hhvm/hphp/runtime/base/array-iterator.cpp:86
#7 0x0000562fddc3292e in HPHP::IterateKV, false> (arrFn=..., adata=0x7f8942faa140) at /build/hhvm/hphp/runtime/base/array-iterator.h:354
#8 HPHP::IterateKVNoInc > (arrFn=..., adata=0x7f8942faa140) at /build/hhvm/hphp/runtime/base/array-iterator.h:364
#9 HPHP::ArrayData::toDArray (this=0x7f8942faa140, copy=copy@entry=true) at /build/hhvm/hphp/runtime/base/array-data.cpp:1112
#10 0x0000562fdebf35c5 in HPHP::Array::toDArray (this=0x7f89b9ffb120) at /build/hhvm/hphp/runtime/base/type-array.h:202
#11 HPHP::HashCollection::toDArray (this=0x7f893cfd8250) at /build/hhvm/hphp/runtime/ext/collections/hash-collection.cpp:106
#12 0x0000562fe3c6437a in ?? ()
#13 0x00007f8a599ff710 in ?? ()
#14 0x00007f8940b519c0 in ?? ()
#15 0x0000562fe3c00018 in ?? ()
#16 0x00007f893c767ca0 in ?? ()
#17 0x0000000000000001 in ?? ()
#18 0x00007f89b97f9d28 in ?? ()
#19 0x00007f8940a941b0 in ?? ()
#20 0x0000562fe3c5e84b in ?? ()
#21 0x00007f89b9ffb1b0 in ?? ()
#22 0x0000562fde333683 in HPHP::jit::enterTC (start=0x7f893cfd8250 "\002") at /build/hhvm/hphp/runtime/vm/jit/enter-tc.cpp:68
```

**Environment**
- Operating system
Ubuntu 18.04
- Installation method
Built from source
- HHVM Version
```
HipHop VM 4.80.6 (rel)
Compiler: remotes/origin/4.80.6-slack-0-g0d1cc2f708e29ce3bffe6bd33ca27ad87d908d04
Repo schema: 368419f649dc5856326d84e9ed95fdef2fbad95f
```

The report centers on AsyncMysqlQueryResult::mapRowsTyped(), row->toDArray(), and the C++ paths in bespoke-array.cpp, array-data-defs.h, array-iterator.cpp, and array-data.cpp. Begin by attempting to reproduce the HHVM 4.80.6 segfault with the supplied loop under Ubuntu 18.04; done means identifying the invalid Map cause and verifying that the loop no longer crashes.

Written by the indexing model from the issue text.