Check for monitor_conn existence before accessing the same

Monitor_conn socket gets closed by wifistatemachine and thus
accessing the same in WiFiMonitor thread results in a crash.
Hence check for the same before accessing the same.

Change-Id: I15024fa5324187c6e8dbd483238bfbeea7c4c016
CRs-Fixed: 590655
Merge pull request #1 from CyanogenMod/cm-12.0

Check for monitor_conn existence before accessing the same