Avoid unintentional passing of insert position from string.unpack

Fixes #1706

CHANGELOG

@@ -8,6 +8,9 @@ o [Windows] Add support for the new loopback behavior in Npcap 0.9983. This
   Adapter to be installed, which was a source of problems for some users.
   [Daniel Miller]
 
+o [NSE][GH#1706] VNC handshakes were failing with insert position out of bounds
+  error. [nnposter]
+
 o [NSE][GH#1720] Function marshall_dom_sid2 in library msrpctypes was not
   correctly populating ID Authority. [nnposter]
 

nselib/vnc.lua

@@ -248,7 +248,7 @@ VNC = {
       end
 
       for i=1, vncsec.count do
-        table.insert( vncsec.types, string.unpack("B", tmp, i) )
+        table.insert( vncsec.types, (string.unpack("B", tmp, i)) )
       end
       self.vncsec = vncsec
     end