Fix the exception issue reported in issue rapid7#7585

Fix the exception by initialize a key variable that caused the exception.

Author: jinq102030

modules/auxiliary/scanner/http/brute_dirs.rb

@@ -89,7 +89,7 @@ def run_host(ip)
 
         # Look for a string we can signature on as well
         if(tcode >= 200 and tcode <= 299)
-
+          emesg = nil
           File.open(datastore['HTTP404Sigs'], 'rb').each do |str|
             if(res.body.index(str))
               emesg = str