Correct a bug in the code for NmapOutputViewer.go_to_host that was put in revision 32936.

The following steps caused the bug with the r32936 code:
    1. Start a scan (say Quick Scan for simplicity) for a certain target (say A)
    2. Wait for scan to complete
    3. Start a scan for a different target (say B).
    4. Wait for scan to complete
    5. Click on host A in the Hosts list on the left (while scan B is still visible on the right)
    6. BOOM!