[SeeRM #8815] Dont try to exploit when generate_payload_exe fails

modules/exploits/multi/http/struts_code_exec_parameters.rb

@@ -120,6 +120,9 @@ class Metasploit3 < Msf::Exploit::Remote
     #Set up generic values.
     payload_exe = rand_text_alphanumeric(4 + rand(4))
     pl_exe = generate_payload_exe
+    if pl_exe.nil?
+      fail_with(Failure::BadConfig, "#{peer} - Failed to generate an EXE payload, please select a correct payload")
+    end
     append = false
     #Now arch specific...
     case target['Platform']