win32/file.c: relative path with drive letter

	* win32/file.c (IS_ABSOLUTE_PATH_P): home directory should not be
	  a relative path regardless a drive letter.   PathIsRelativeW
	  returns FALSE on such path.  [ruby-core:86356] [Bug #14638]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@64421 b2dd03c8-39d4-4d8f-98ff-823fe69b080e