add yml check

[fedya]

in case of

Thu Nov 26 14:47:04 2020 LOG [INFO] | spec_name is konversation.spec
Thu Nov 26 14:47:04 2020 LOG [INFO] | single spec in repo, check passed
Thu Nov 26 14:47:04 2020 LOG [INFO] | Traceback (most recent call last):
Thu Nov 26 14:47:04 2020 LOG [INFO] |   File "/etc/builder-c/mdv/build-rpm.py", line 495, in <module>
Thu Nov 26 14:47:04 2020 LOG [INFO] |     download_yml(build_package + '/' + '.abf.yml')
Thu Nov 26 14:47:04 2020 LOG [INFO] |   File "/etc/builder-c/mdv/build-rpm.py", line 120, in download_yml
Thu Nov 26 14:47:04 2020 LOG [INFO] |     for key, value in data['sources'].items():
Thu Nov 26 14:47:04 2020 LOG [INFO] | AttributeError: 'NoneType' object has no attribute 'items'
Thu Nov 26 14:47:05 2020 Main [INFO] | Build is over with status BUILD_FAILED.

need to validate yml