Socket: Fix cascading crash when Socket.run() crashes.

When a driver's run() method crashes, supybot.drivers.run() marks it
as dead and sets its 'irc' attribute to None.

This would be fine for "normal" independent drivers (like Socket used
to be), because this driver would never be called again.

But now that we use select(), some other thread may hold a reference
to this driver in a select() call frame, and call the dead driver's
'_read()' method when there is data to be read from the socket.

There is already a safeguard in '_read()' in the case the socket could
be read from, but this safeguard was missing from _handleSocketError.
This caused the "live" driver's select() to crash, which propagagated
to its run(), which caused the driver to be marked as dead, etc.

Eventually, all drivers could die, and we end up with the dreadful
"Schedule is the only remaining driver, why do we continue to live?"
in an infinite loop.

src/drivers/Socket.py

@@ -114,6 +114,13 @@ class SocketDriver(drivers.IrcDriver, drivers.ServersMixin):
             except:
                 pass
             self.connected = False
+            if self.irc is None:
+                # This driver is dead already, but we're still running because
+                # of select() running in an other driver's thread that started
+                # before this one died and stil holding a reference to this
+                # instance.
+                # Just return, and we should never be called again.
+                return
             self.scheduleReconnect()
         else:
             log.debug('Got EAGAIN, current count: %s.', self.eagains)
@@ -208,6 +215,8 @@ class SocketDriver(drivers.IrcDriver, drivers.ServersMixin):
 
                 msg = drivers.parseMsg(line)
                 if msg is not None and self.irc is not None:
+                    # self.irc may be None if this driver is already dead,
+                    # see comment in _handleSocketError
                     self.irc.feedMsg(msg)
         except socket.timeout:
             pass