Copy check for data_header to avoid crash

Variable was used but out of scope.

modules/exploits/multi/http/struts2_namespace_ognl.rb

@@ -351,6 +351,11 @@ class MetasploitModule < Msf::Exploit::Remote
   end
 
   def send_payload_oneshot(payload)
+    data_header = datastore['HEADER']
+    if data_header.empty?
+      fail_with(Failure::BadConfig, "HEADER parameter cannot be blank when sending a payload")
+    end
+
     random_filename = datastore['TEMPFILE']
 
     # d = payload data