do not allow cursor movements for non-valid cursors

src/core/PieceTable.cc

@@ -124,6 +124,8 @@ PieceTable::Cursor::Cursor(PieceTable * pt) : iterator(pt)
 
 bool PieceTable::Cursor::go_start_of_line()
 {
+    if (!iterator.valid())
+        return false;
     bool rv = false;
     while (iterator.valid() &&
             (!iterator.piece->prev->eol()) &&
@@ -141,6 +143,8 @@ bool PieceTable::Cursor::go_start_of_line()
 
 bool PieceTable::Cursor::go_end_of_line()
 {
+    if (!iterator.valid())
+        return false;
     bool rv = false;
     while (iterator.valid() &&
             (!iterator.piece->eol()) &&
@@ -159,6 +163,8 @@ bool PieceTable::Cursor::go_end_of_line()
 
 bool PieceTable::Cursor::go_up(int n, uint32_t desired_column)
 {
+    if (!iterator.valid())
+        return false;
     Piece * p = prev_line();
     if (p == nullptr)
     {
@@ -174,6 +180,8 @@ bool PieceTable::Cursor::go_up(int n, uint32_t desired_column)
 
 bool PieceTable::Cursor::go_down(int n, uint32_t desired_column)
 {
+    if (!iterator.valid())
+        return false;
     Piece * p = next_line();
     if (p == nullptr)
     {
@@ -189,6 +197,8 @@ bool PieceTable::Cursor::go_down(int n, uint32_t desired_column)
 
 bool PieceTable::Cursor::go_left(int n)
 {
+    if (!iterator.valid())
+        return false;
     if (is_start_of_line())
     {
         return false;
@@ -209,6 +219,8 @@ bool PieceTable::Cursor::go_left(int n)
 
 bool PieceTable::Cursor::go_right(int n)
 {
+    if (!iterator.valid())
+        return false;
     if (is_end_of_line())
     {
         return false;