defmacro: Error: runtime error: invalid memory address or nil pointer dereference

Given this 
```
defmacro:
  name: ifdef
  args: $varname
  body: 12
---
ifdef: {quote: Foob}
```

Fails with:
```
Error: runtime error: invalid memory address or nil pointer dereference
github.com/birchb1024/goyamp.(*Expander).ExpandStream.func1
	/Users/bill.birch/wo/github.com/birchb1024/goyamp/api.go:68

```