Land rapid7#8690, Fix rapid7#8636, [] for NilClass in session.fs.file.download_file

Author: bwatters-r7

lib/rex/post/meterpreter/extensions/stdapi/fs/file.rb

@@ -373,7 +373,7 @@ def File.download_file(dest_file, src_file, opts = nil, &stat)
     end
 
     # Keep transferring until EOF is reached...
-    block_size = opts['block_size'] || 1024 * 1024
+    block_size = (opts && opts['block_size']) || 1024 * 1024
     begin
       if tries
         # resume when timeouts encountered