ed: k0 is invalid (#921)

* Avoid saving address 0 as a "marked address"
* When edMark() was first written I didn't pick up that this usage was not allowed in other versions of ed

bin/ed

@@ -346,6 +346,7 @@ sub edMark {
     my $ad = $adrs[1];
     $ad = $adrs[0] unless defined $ad;
     $ad = $CurrentLineNum unless defined $ad;
+    return E_ADDRBAD if $ad == 0;
     $marks{$c} = $ad;
     return;
 }