Security issue - can kill server by using a malformed operations header
Hello,

There have been issues like this one in the past, here's a new one I have found.

When sending a multipart request using some arbitrary REST client I can kill the express server. Namely, if the `variants` field in the operations header has a value other than an object, the server dies. A normal operations header would look something like this:

`{"operationName": null, "variables": {"images": [null, null]}, "query": "mutation ($images: [Upload!]!) {  uploadImages(images: $images) { id image } }"}`

If you change the value of `variables` (`{"images": [null, null]}`) to any of the following, sending such request will kill the server:

- any string value - "whateverXYZ214213"
- any number - 123
- null

Array `[]` and object `{}` work fine. The error that is thrown is this:

```
[0] TypeError: Cannot create property 'images' on string '123'
[0]     at set (/home/juona/dev/podbase/node_modules/object-path/index.js:118:28)
[0]     at set (/home/juona/dev/podbase/node_modules/object-path/index.js:124:14)
[0]     at set (/home/juona/dev/podbase/node_modules/object-path/index.js:104:16)
[0]     at Function.objectPath.set (/home/juona/dev/podbase/node_modules/object-path/index.js:157:14)
[0]     at Busboy.parser.on (/home/juona/dev/podbase/node_modules/graphql-upload/lib/processRequest.js:173:30)
[0]     at Busboy.emit (events.js:182:13)
[0]     at Busboy.emit (/home/juona/dev/podbase/node_modules/graphql-upload/node_modules/busboy/lib/main.js:37:33)
[0]     at PartStream.onEnd (/home/juona/dev/podbase/node_modules/graphql-upload/node_modules/busboy/lib/types/multipart.js:262:15)
[0]     at PartStream.emit (events.js:187:15)
[0]     at Dicer.onPart (/home/juona/dev/podbase/node_modules/graphql-upload/node_modules/busboy/lib/types/multipart.js:120:13)
[0]     at Dicer.emit (events.js:182:13)
[0]     at Dicer.emit (/home/juona/dev/podbase/node_modules/graphql-upload/node_modules/dicer/lib/Dicer.js:79:35)
[0]     at Dicer._oninfo (/home/juona/dev/podbase/node_modules/graphql-upload/node_modules/dicer/lib/Dicer.js:180:12)
[0]     at SBMH.<anonymous> (/home/juona/dev/podbase/node_modules/graphql-upload/node_modules/dicer/lib/Dicer.js:126:10)
[0]     at SBMH.emit (events.js:182:13)
[0]     at SBMH._sbmh_feed (/home/juona/dev/podbase/node_modules/streamsearch/lib/sbmh.js:188:10)

```