Segmentation fault from node-webrtc when accessing RTCPeerConnection.localDescription

**What version of this package are you using?**

* v9.11.0

**What operating system, Node.js, and npm version?**

I haven't tested this in other environments.

* macOS 11.3
* Node.js v14.16.1
* npm 6.14.12
* wrtc 0.4.7

**What happened?**

When creating, connecting, and destroying several SimplePeer instances in Node.js, `wrtc` crashes with a segmentation fault:

```
PID 4228 received SIGSEGV for address: 0x0
0 segfault-handler.node 0x00000001046bdfb0 _ZL16segfault_handleriP9__siginfoPv + 304
1 libsystem_platform.dylib 0x00007fff203b3d7d _sigtramp + 29
2 ??? 0x0000000200583232 0x0 + 8595714610
3 wrtc.node 0x00000001151e4e86 _ZNK6webrtc22JsepSessionDescription8ToStringEPNSt3__112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEE + 38
4 wrtc.node 0x0000000115028fe2 _ZN11node_webrtc9ConverterIPKN6webrtc27SessionDescriptionInterfaceENS_25RTCSessionDescriptionInitEE7ConvertES4_ + 82
5 wrtc.node 0x0000000115029b60 _ZN11node_webrtc9ConverterINSt3__14pairIN4Napi3EnvEPKN6webrtc27SessionDescriptionInterfaceEEENS3_5ValueEE7ConvertES9_ + 48
6 wrtc.node 0x000000011510416f _ZN11node_webrtc17RTCPeerConnection19GetLocalDescriptionERKN4Napi12CallbackInfoE + 111
7 wrtc.node 0x0000000115119bdb _ZZN4Napi10ObjectWrapIN11node_webrtc17RTCPeerConnectionEE29InstanceGetterCallbackWrapperEP10napi_env__P20napi_callback_info__ENKUlvE_clEv + 139
8 wrtc.node 0x0000000115119a9a _ZN4Napi10ObjectWrapIN11node_webrtc17RTCPeerConnectionEE29InstanceGetterCallbackWrapperEP10napi_env__P20napi_callback_info__ + 42
9 node 0x000000010006b94a _ZN6v8impl12_GLOBAL__N_123FunctionCallbackWrapper6InvokeERKN2v820FunctionCallbackInfoINS2_5ValueEEE + 122
10 node 0x0000000100a0bacd Builtins_CallApiCallback + 173
[1] 4227 segmentation fault npm start
```

**Reproduction case**

I created a repository with a demo, and also pasted example code below. Unfortunately it's not deterministic, but when running it in 4 windows, it crashes before 1000 iterations in at least one of them.

https://github.com/dguenther/simple-peer-issue-demo

Click to expand example code block

```ts
require('segfault-handler').registerHandler('segfault.log')
const SimplePeer = require('simple-peer')
const wrtc = require('wrtc')

const LOOP_TIME_MS = 70

function getRandomInt(min, max) {
min = Math.ceil(min);
max = Math.floor(max);
return Math.floor(Math.random() * (max - min + 1)) + min;
}

let iteration = 0

const initiators = []
const recipients = []

async function eventLoop() {
console.log(`Iteration ${++iteration}`)

while (initiators.length > 20) {
const conn = initiators.splice(getRandomInt(0, initiators.length - 1), 1)[0]
conn.destroy()
}

while (recipients.length > 20) {
const conn = recipients.splice(getRandomInt(0, initiators.length - 1), 1)[0]
conn.destroy()
}

for (let i = 0; i < 4; i++) {
const recip = new SimplePeer({ initiator: false, wrtc })
const init = new SimplePeer({ initiator: true, wrtc })

recip.on('signal', (signal) => {
if (!init.destroyed) init.signal(signal)
})
init.on('signal', (signal) => {
if (!recip.destroyed) recip.signal(signal)
})

initiators.push(init)
recipients.push(recip)
}

setTimeout(eventLoop, LOOP_TIME_MS)
}

eventLoop()
```

**What did you expect to happen?**

No crash 😄 Since ultimately it should be node-webrtc's responsibility to manage itself without crashing, I've created an issue here: https://github.com/node-webrtc/node-webrtc/issues/696

However, I noticed that removing `this._pc.localDescription` from this line fixes the crash:

https://github.com/feross/simple-peer/blob/d972548299a50f836ca91c36e39304ef0f9474b7/index.js#L618

[Several](https://blog.mozilla.org/webrtc/perfect-negotiation-in-webrtc/) [WebRTC examples](https://developer.mozilla.org/en-US/docs/Web/API/WebRTC_API/Simple_RTCDataChannel_sample) seem to pass localDescription to the other peer rather than passing the offer itself, so I wasn't sure if there was a reason for that, or if this is a viable workaround.

**Are you willing to submit a pull request to fix this bug?**

👍 Yep, if one is necessary.