ed: runtime error on empty file (#560)

* Fix the assumption in edEdit() that >0 bytes will be read from input file; it might be empty
* Error discovered with "perl ed some.txt" then "1r EMPTYFILE" ... Modification of non-creatable array value attempted, subscript -1 at ed line 722.
* Follow GNU ed and print the number of characters read (0)
* The default calculation of $CurrentLineNum was correct for all cases of insert mode, so use that

bin/ed

@@ -718,6 +718,11 @@ sub edEdit {
         edWarn(E_CLOSE);
         return 0;
     }
+    if ($chars == 0) {
+        $UserHasBeenWarned = 0;
+        print "0\n" unless $SupressCounts;
+        return 1;
+    }
     if (substr($tmp_lines[-1], -1, 1) ne "\n") {
         $tmp_lines[-1] .= "\n";
         $chars++;
@@ -729,15 +734,12 @@ sub edEdit {
     if ($InsertMode) {
         if (maxline() != 0 && $adrs[0] == maxline()) {
             push(@lines,@tmp_lines);
-            $CurrentLineNum = maxline();
         } elsif ($adrs[0] == 0) {
             splice @lines, 1, 0, @tmp_lines;
-            $CurrentLineNum = scalar(@tmp_lines);
         } else {
             splice @lines, $adrs[0] + 1, 0, @tmp_lines;
-            $CurrentLineNum = $adrs[0] + scalar(@tmp_lines);
         }
-
+        $CurrentLineNum = $adrs[0] + scalar(@tmp_lines);
         $NeedToSave = 1;
     } else {
         if ($NeedToSave && $QuestionsMode && !$UserHasBeenWarned) {