kernel: Avoid page faults when reading from a slave pseudoterminal after the master is destroyed

kernel/src/fs/devices/SlavePTY.cpp

@@ -36,6 +36,8 @@ Result<usize> SlavePTY::write(const u8* buf, usize, usize length)
 
 bool SlavePTY::will_block_if_read() const
 {
+    if (!m_master) return false;
+
     if (!m_master->is_canonical()) return m_buffer.is_empty();
     else
         return !m_master->m_lines.size();