Land rapid7#4739, @zeroSteiner's change to prepend_sleep powershell option check

Author: jvazquez-r7

lib/msf/core/exploit/powershell.rb

@@ -296,8 +296,8 @@ def cmd_psh_payload(pay, payload_arch, opts = {})
     if opts[:prepend_sleep]
       if opts[:prepend_sleep].to_i > 0
         psh_payload = "Start-Sleep -s #{opts[:prepend_sleep]};" << psh_payload
-      else
-        vprint_error('Sleep time must be greater than 0 seconds')
+      elsif opts[:prepend_sleep].to_i < 0
+        vprint_error('Sleep time must be greater than or equal to 0 seconds')
       end
     end