YAML: Fix crash in the skip method of KeyValueNode class.

This commit changes the 'skip' method in the 'KeyValueNode' class
to ensure that it doesn't dereference a null pointer when calling 
the 'skip' method of its value child node. It also adds a unittest
that ensures that the crash doesn't occur.

This change is motivated by a patch that implements parsing
of YAML block scalars (http://reviews.llvm.org/D9503), as one
of the unittests in that patch triggered this problem.

llvm-svn: 236669