Prevent segfault on malformed yaml with array keys

The simple fix is to check for null pointer on the SCALAR_TAG_IS macros. But
I see absolutely no sense in returning serialized data from
convert_to_char() which is used only for keys. So I've just dropped the
serialization block, and handle the null return.

Bug: 64694
Add test for bug #64694

Add a test to validate the expected behavior of yaml_parse() with the patch to
fix bug #64694 applied.

Bug: 64694
Revert "Prevent segfault on malformed yaml with array keys"

This reverts commit 86c9a85626f7528e897d3871e703e4fe4452ae94. The fix provided
in that patch does prevent the PHP segfault bug, but it does so by violating
the YAML 1.1 specification (http://yaml.org/spec/1.1/#id933629).
Guard against crash caused by treating non-scalar event as scalar

Bug: 64694
Merge branch 'pull-request/4'