[System.Threading] Be proactive avoiding possible ArgumentNullException

This way the exception is not generated for every WebRequest in the server,
which should imply better performance.

For the record, the stacktrace I was getting by using --trace=E:all, is:

[0x7fc6a6de1700:] EXCEPTION handling: System.ArgumentNullException: null handle
Parameter name: waitHandles

"Threadpool worker" tid=0x0x7fc6a6de1700 this=0x0x7fc6a62573c0 thread handle 0x4d6 state : not waiting owns ()
  at System.Threading.WaitHandle.CheckArray (System.Threading.WaitHandle[],bool) [0x00085] in /home/andres1304/Code/system_mono/mono/mcs/class/corlib/System.Threading/WaitHandle.cs:78
  at System.Threading.WaitHandle.WaitAny (System.Threading.WaitHandle[],System.TimeSpan,bool) [0x00000] in /home/andres1304/Code/system_mono/mono/mcs/class/corlib/System.Threading/WaitHandle.cs:211
  at System.Threading.RegisteredWaitHandle.Wait (object) [0x00019] in /home/andres1304/Code/system_mono/mono/mcs/class/corlib/System.Threading/RegisteredWaitHandle.cs:72
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void__this___object (object,intptr,intptr,intptr) <IL 0x00052, 0xffffffff>