avoid NPE/fatal exception if wpa_supplicant fails

If wpa_supplicant dies unexpectedly (for example due to
broken wpa_supplicant.conf), WifiStateService tries to
stop DHCP. If no DhcpHandler has been created yet, this
causes an NPE which is not caught. This leads to a fatal
exception in ConnectivityThread, finally causing Zygote
to shutdown and reinit in a loop.
These additional null checks fix the problem.

Change-Id: Id7d9212225c2b5f8d1dadcb9049b03b6e28550ea