Trailing scalar in block mapping generates no error at end of document

Example YAML document:

```yaml
---
foo: bar
not-a-key
```

This is incorrectly parsed successfully with `not-a-key` treated as a key in the map with a null value.

If there is anything after the scalar, e.g.:

```yaml
---
foo: bar
not-a-key
# A comment
```

Then a ParserException is thrown:

> what():  yaml-cpp: error at line 3, column 1: end of map not found