ls: runtime error for non-existent file argument

* When testing ls today I discovered the error: Can't use an undefined value as a HASH reference at ls line 525.
* Test case: "perl ls xxx ar a.x pig" where files xxx and a.x don't exist
* I broke this in the previous commit around where DirEntries() is called for Files-list
* Add correct return value guard to fix error
* DirEntries() prints a warning and returns an empty list if stat() fails on its file argument
* Derive sorted-list from attrs-hash to avoid listing filenames where stat() failed

%perl ls  xxx ar a.x pig
ls: can't access 'xxx': No such file or directory
ls: can't access 'a.x': No such file or directory
ar   pig

Author: mknos

bin/ls

@@ -522,9 +522,10 @@ if ($#ARGV < 0) {
 		my %attrs;
 		foreach (@Files) {
 			my @ret = DirEntries(\%Options, $_);
+			next unless @ret; # stat() failed
 			%attrs = (%attrs, %{ $ret[-1] });
 		}
-		my @sorted = Order(\%Options, \%Attributes, @Files);
+		my @sorted = Order(\%Options, \%Attributes, keys %attrs);
 		List('.', \%Options, 0, 0, @sorted, \%attrs);
 	}
 	for my $Arg (@Dirs) {