Fix rapid7#4119 - SMB lost search ID (sid) in find_first method

This will fix issue rapid7#4119. A bug in the find_first method in rex
SMB.

When the SMB client requests a TRANS2_FIND_FIRST2 for retriving
information about what items a directory has, the server returns
a response that contains an SID - a search identifier for the
transaction. If the SMB client wants more data, it must send a
TRANS2_FIND_NEXT2 request with the same SID. And then the server
will continue sending more until it runs out.

The root cause of this bug is that after the TRANS2_FIND_FIRST2
request is sent, our SMB's find_first method forgets the SID at
the end of the loop (out of scope).

Author: wchen-r7

lib/rex/proto/smb/client.rb

@@ -1872,6 +1872,7 @@ def queryfs_fs_attribute
 
   # Enumerates a specific path on the mounted tree
   def find_first(path)
+    sid = nil
     files = { }
     parm = [
       26,  # Search for ALL files