Detect malformed YAML sequence in yaml::Input::beginSequence()

When reading a yaml::SequenceTraits object, YAMLIO does not report an
error if the yaml item is not a sequence. Instead, YAMLIO reads an
empty sequence. For example:

---
seq:
    foo: 1
    bar: 2
...

If `seq` is a SequenceTraits object, then reading the above yaml will
yield `seq` as an empty sequence.

Fix this to report an error for the above mapping ("not a sequence")

Patch by William Fisher. Thanks!

llvm-svn: 230976