Add sanity check for NUM_REQUESTS

Author: jhart-r7

lib/msf/core/auxiliary/drdos.rb

@@ -17,6 +17,13 @@ def initialize(info = {})
       ], self.class)
   end
 
+  def setup
+    super
+    if spoofed? && datastore['NUM_REQUESTS'] < 1
+      raise Msf::OptionValidateError.new(['NUM_REQUESTS']), 'The number of requests must be >= 1'
+    end
+  end
+
   def prove_amplification(response_map)
     vulnerable = false
     proofs = []