Possible bug 3020 fix: Segmentation fault: nameservers[vc->ns].vc = NULL

Possible bug 3020 fix: Segmentation fault: nameservers[vc->ns].vc = NULL

Prevent idnsVCClosed segfaults during shutdown or reconfiguration (at least).

idnsShutdown() schedules comm_close and then frees nameservers[] by
calling idnsFreeNameservers. The closing handler tried to access freed
nameservers[]. The patch prevents access to the freed nameservers[]
array in idnsVCClosed and other functions.

TODO: Nameservers[] array management should be rewritten. The array
should not be freed while there are nameservers using it. It should be
freed when the last entry is gone.

From: 3p1-rock r9583