ASSERTION (!VarIs<RecyclableObject>(instance) ? 1 : ((RecyclableObject*)instance)->GetScriptContext()->GetLibrary() == library) failed in Js::JavascriptOperators::IsUndefinedObject

###### ChakraCore version:

```
Checked revision: a247f4f3c
Build command: ./build.sh --debug
```

###### OS:

```
Linux-4.15.0-43-generic-x86_64-with-Ubuntu-18.04-bionic
```

###### Test case:

```js
function MyRepl($) {
$.substring($, WScript.LoadScript(0, "samethread"))
}
var str = String("asdfasdfGg");
WScript(str.replace(/d/g, MyRepl));
```

###### Backtrace:

```text
ASSERTION 9928: (ChakraCore/lib/Runtime/Language/JavascriptOperators.cpp, line 10952) !VarIs(instance) ? TRUE : ((RecyclableObject*)instance)->GetScriptContext()->GetLibrary() == library
Failure: (!VarIs(instance) ? 1 : ((RecyclableObject*)instance)->GetScriptContext()->GetLibrary() == library)

Thread 1 "ch" received signal SIGILL, Illegal instruction.
0x00007ffff35dc30f in Js::JavascriptOperators::IsUndefinedObject (instance=0x7ffff7f04420, library=0x7ffff7f18000)
at ChakraCore/lib/Runtime/Language/JavascriptOperators.cpp:10952
10952 Assert(!VarIs(instance) ? TRUE : ((RecyclableObject*)instance)->GetScriptContext()->GetLibrary() == library );
(gdb) bt
#0 0x00007ffff35dc30f in Js::JavascriptOperators::IsUndefinedObject (instance=0x7ffff7f04420, library=0x7ffff7f18000)
at ChakraCore/lib/Runtime/Language/JavascriptOperators.cpp:10952
#1 0x00007ffff35dc950 in Js::JavascriptOperators::IsUndefinedObject (instance=0x7ffff7f04420, scriptContext=0x5555561b2a88)
at ChakraCore/lib/Runtime/Language/JavascriptOperators.cpp:10947
#2 0x00007ffff39d9185 in Js::JavascriptString::EntrySubstring (function=0x7ffff7eaf440, callInfo=...)
at ChakraCore/lib/Runtime/Library/JavascriptString.cpp:1949
#3 0x00007ffff3ab445e in amd64_CallFunction ()
at ChakraCore/lib/Runtime/Library/amd64/JavascriptFunctionA.S:100
#4 0x00007ffff37c06ee in Js::JavascriptFunction::CallFunction (function=0x7ffff7eaf440,
entryPoint=0x7ffff39d8e80 , args=...,
useLargeArgCount=false) at ChakraCore/lib/Runtime/Library/JavascriptFunction.cpp:1342
#5 0x00007ffff3583d4c in Js::InterpreterStackFrame::OP_CallCommon > > __unaligned>(Js::OpLayoutDynamicProfile > > __unaligned const __unaligned*, Js::RecyclableObject*, unsigned int, Js::AuxArray const*) (
this=0x7fffffff81c0, playout=0x7ffff7f4d44c, function=0x7ffff7eaf440, flags=16, spreadIndices=0x0)
at ChakraCore/lib/Runtime/Language/InterpreterStackFrame.cpp:3858
#6 0x00007ffff35838ae in Js::InterpreterStackFrame::OP_ProfileCallCommon > > __unaligned>(Js::OpLayoutDynamicProfile > > __unaligned const __unaligned*, Js::RecyclableObject*, unsigned int, unsigned short, unsigned int, Js::AuxArray const*) (this=0x7fffffff81c0, playout=0x7ffff7f4d44c, function=0x7ffff7eaf440, flags=0, profileId=0,
inlineCacheIndex=0, spreadIndices=0x0) at ChakraCore/lib/Runtime/Language/InterpreterStackFrame.cpp:3902
#7 0x00007ffff354be58 in Js::InterpreterStackFrame::OP_ProfiledCallIWithICIndex > >(Js::OpLayoutDynamicProfile > > const __unaligned*) (this=0x7fffffff81c0, playout=0x7ffff7f4d44c)
at ChakraCore/lib/Runtime/./Language/InterpreterStackFrame.h:503
#8 0x00007ffff33fbed6 in Js::InterpreterStackFrame::ProcessProfiled (this=0x7fffffff81c0)
at ChakraCore/lib/Runtime/Language/InterpreterHandler.inl:87
#9 0x00007ffff338d59d in Js::InterpreterStackFrame::Process (this=0x7fffffff81c0)
at ChakraCore/lib/Runtime/Language/InterpreterStackFrame.cpp:3357
#10 0x00007ffff338c307 in Js::InterpreterStackFrame::InterpreterHelper (function=0x7ffff7f066e0, args=...,
returnAddress=0x7ffff7e60f9a, addressOfReturnAddress=0x7fffffff8728, asmJsReturn=0x0)
at ChakraCore/lib/Runtime/Language/InterpreterStackFrame.cpp:2051
#11 0x00007ffff338b4d8 in Js::InterpreterStackFrame::InterpreterThunk (layout=0x7fffffff8740)
at ChakraCore/lib/Runtime/Language/InterpreterStackFrame.cpp:1763
#12 0x00007ffff7e60f9a in ?? ()
#13 0x00007fffffff8770 in ?? ()
#14 0x00007ffff3ab445e in amd64_CallFunction ()
at ChakraCore/lib/Runtime/Library/amd64/JavascriptFunctionA.S:100
```

^{Found by [Fuzzinator](http://fuzzinator.readthedocs.io/) with [grammarinator](https://github.com/renatahodovan/grammarinator).}