Make Cursor::go_start_of_line() and Cursor::go_end_of_line() safe for empty pieces

src/core/PieceTable.cc

@@ -150,14 +150,17 @@ PieceTable::Cursor::Cursor(PieceTable * pt) : iterator(pt)
 
 void PieceTable::Cursor::go_start_of_line()
 {
-    if (!iterator.valid())
+    Iterator last_iterator = iterator;
-        return;
+    for (;;)
-    while (iterator.valid() &&
-            (!iterator.piece->prev->eol()) &&
-            (iterator.piece->prev != iterator.piece_table->start_piece))
     {
         iterator.go_prev_piece();
+        if (iterator.piece_eol() || !iterator.valid())
+        {
+            break;
+        }
+        last_iterator = iterator;
     }
+    iterator = last_iterator;
     iterator.offset = 0u;
     init_column();
 }
@@ -173,12 +176,25 @@ bool PieceTable::Cursor::check_go_start_of_line()
 void PieceTable::Cursor::go_end_of_line(bool allow_on_eol)
 {
     if (!iterator.valid())
-        return;
-    while (iterator.valid() &&
-            (!iterator.piece->eol()) &&
-            (iterator.piece->next != iterator.piece_table->end_piece))
     {
-        iterator.go_next_piece();
+        return;
+    }
+    for (;;)
+    {
+        if (iterator.piece_eol())
+        {
+            break;
+        }
+        Iterator move_iterator = iterator;
+        move_iterator.go_next_piece();
+        if (move_iterator.valid())
+        {
+            iterator = move_iterator;
+        }
+        else
+        {
+            break;
+        }
     }
     iterator.go_end_of_piece();
     calculate_column();