parse fail with here document with variable

Parse fails for entire document when you have a here document inside an `if`, that contains a variable as the first expression.

MWE:
```bash
if true; then
  a_string='some stuff to cat'
  cat <<XXX
${a_string}
XXX
fi
```

Issue does not occur if:
- not inside an `if`
- there is anything except newlines before `${a_string}`, including whitespace