fix Buffer::clear() to update iterators

src/core/Buffer.cc

@@ -461,3 +461,13 @@ void Buffer::redo()
         m_current_change_operation_index = child_index;
     }
 }
+
+void Buffer::clear()
+{
+    m_gap_buffer->clear();
+    for (auto iterator : m_iterators)
+    {
+        auto b = begin();
+        *iterator = b;
+    }
+}

src/core/Buffer.h

@@ -119,7 +119,7 @@ public:
         m_cursors.push_back(iterator);
         return iterator;
     }
-    void clear() { m_gap_buffer->clear(); }
+    void clear();
     auto get_string() { return m_gap_buffer->get_string(); }
     void set_string(const std::string & s) { m_gap_buffer->set_string(s); }
     size_t size() const { return m_gap_buffer->size(); }