fix bug in line_gap.sml's 'deleteRightFromHere' function: when we reach end of buffer, return with current index and current line instead of setting these to 0, and return input leftString and leftLines when we reach the end, instead of an empty list for these fields, because all of our text content is in those values and it would disappear if we return empty lists instead

src/line_gap.sml

@@ -1336,14 +1336,14 @@ struct
               }
           end
       | (_, _) =>
-          { idx = 0
+          { idx = origIdx
           , textLength = textLength
-          , line = 0
+          , line = origLine
           , lineLength = lineLength
-          , leftStrings = []
-          , leftLines = []
-          , rightStrings = rightStrings
-          , rightLines = rightLines
+          , leftStrings = leftStrings
+          , leftLines = leftLines
+          , rightStrings = []
+          , rightLines = []
           }
 
     fun moveRightAndDelete