TTY: MasterPTY should fail to ioctl() if slave is gone

Just fail with EIO in that case.

Kernel/TTY/MasterPTY.cpp

@@ -98,6 +98,8 @@ void MasterPTY::close()
 
 int MasterPTY::ioctl(FileDescription& description, unsigned request, unsigned arg)
 {
+    if (!m_slave)
+        return -EIO;
     if (request == TIOCSWINSZ || request == TIOCGPGRP)
         return m_slave->ioctl(description, request, arg);
     return -EINVAL;