[os_xml] fix two possible nullpointer dereferences (found by scan-build) and use nonnull attributes for static analysers
[os_xml] fix -Wsign-conversion
revert back some files for smaller merge
[tests] build tests for os_xml after merge cleanup
[tests] create coverage make target
[tests] add some more tests
[os_xml] astyle format

formatting with
astyle --indent=spaces=4 --indent-switches --lineend=linux --style=stroustrup --pad-oper --add-brackets --align-pointer=name os_xml/*.c
astyle --indent=spaces=4 --indent-switches --lineend=linux --style=stroustrup --pad-oper --add-brackets --align-pointer=name os_xml/*.h

see https://github.com/ossec/ossec-docs/blob/fae75e0beb6201d7edec5d65d30d657c565cfedc/docs/oRFC/orfc-2.rst