Cannot create new Jimp from Buffer

# Expected Behavior

Would like to create a new Jimp object from a buffer

# Current Behavior

with this code:
```
new Jimp({
data: Buffer.from(
``
),
width: width,
height: height
});
```
I get the error message:
```
Error: w and h must be numbers
```

## Steps to Reproduce

run this code :
```
new Jimp({
data: Buffer.from(
``
),
width: width,
height: height
});
```

## Context

Basically what I'm trying to do is generate a Jimp object with rounded corners to use as a mask on another image (i.e. with `myImage.composite(maskFromBuffer,0,0)`

- Jimp Version: 0.9.8
- Operating System: MacOS
- Node version:v12.17.0