Look ahead before testing for EOF.
This fixes panics in saphyr's test_multiline_trailing_newline and
test_multiline_leading_newline tests, in which `self.input.next_is_z`
would be called on an empty buffer and panic in `peek`
These are corner cases not tested by the `yaml-test-suite`.
Parsing for the reported input has been fixed, as well as other
similar-looking inputs which make use of nested implicit flow mappings
and implicit null keys and values.
Fixes#1.
