WASHINGTON — Sales of previously occupied homes fell in December for the first time in four months as many would-be buyers were frustrated by a lack of available houses, which fell to the lowest level in more than two decades. Existing home sales dropped 4.6 percent last month from November, to a seasonally adjusted annual rate of nearly 6.2 million, the National Association of Realtors said Thursday. The demand for homes remains healthy, the group said, with median prices jumping nearly 16 percent from a year ago to $358,000.