HexEditor: Fix out of bounds cursor

Fixing out of bounds cursor in three different cases:
- when the buffer is empty
- when loading new files
- when entering values at the end of the buffer

Applications/HexEditor/HexEditor.cpp

@@ -67,6 +67,8 @@ void HexEditor::set_buffer(const ByteBuffer& buffer)
 {
     m_buffer = buffer;
     set_content_length(buffer.size());
+    m_position = 0;
+    m_byte_position = 0;
     update();
     update_status();
 }
@@ -417,6 +419,10 @@ void HexEditor::keydown_event(GUI::KeyEvent& event)
 void HexEditor::hex_mode_keydown_event(GUI::KeyEvent& event)
 {
     if ((event.key() >= KeyCode::Key_0 && event.key() <= KeyCode::Key_9) || (event.key() >= KeyCode::Key_A && event.key() <= KeyCode::Key_F)) {
+        if (m_buffer.is_empty())
+            return;
+        ASSERT(m_position >= 0);
+        ASSERT(m_position < m_buffer.size());
 
         // yes, this is terrible... but it works.
         auto value = (event.key() >= KeyCode::Key_0 && event.key() <= KeyCode::Key_9)
@@ -429,7 +435,8 @@ void HexEditor::hex_mode_keydown_event(GUI::KeyEvent& event)
             m_byte_position++;
         } else {
             m_buffer.data()[m_position] = (m_buffer.data()[m_position] & 0xF0) | value; // save the first 4 bits, OR the new value in the last 4
-            m_position++;
+            if (m_position + 1 < m_buffer.size())
+                m_position++;
             m_byte_position = 0;
         }
 
@@ -441,10 +448,17 @@ void HexEditor::hex_mode_keydown_event(GUI::KeyEvent& event)
 
 void HexEditor::text_mode_keydown_event(GUI::KeyEvent& event)
 {
+    if (m_buffer.is_empty())
+        return;
+    ASSERT(m_position >= 0);
+    ASSERT(m_position < m_buffer.size());
+
     m_tracked_changes.set(m_position, m_buffer.data()[m_position]);
     m_buffer.data()[m_position] = (u8)event.text().characters()[0]; // save the first 4 bits, OR the new value in the last 4
-    m_position++;
+    if (m_position + 1 < m_buffer.size())
+        m_position++;
     m_byte_position = 0;
+
     update();
     update_status();
     did_change();