removed invalid nullptr checks.
New allocation doesn't gurantee that returns nullptr when it's failed.
It's useless on the modern compliers and our policy respects it.
We can replace the system logger method by changing single line print source in common,
This also helps to remove the THORVG_LOG_ENABLED macro from each use-cases.
TVGLOG(): To print the hint & tip messages for users.
TVGERR(): To print the error message for debugging.
@Issues: https://github.com/Samsung/thorvg/issues/36