Fix for two RTSP request assembly defects introduced in r37474:

* Adds missing header-terminating empty line to the request string
* Rectifies a code crash when the header table is empty
Fixes #1781, closes #1796

CHANGELOG

@@ -10,6 +10,9 @@ 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#1781][GH#1796] The RTSP library was not correctly generating request
+  strings. [nnposter]
+
 o [NSE][GH#1706] VNC handshakes were failing with insert position out of bounds
   error. [nnposter]
 

nselib/rtsp.lua

@@ -80,10 +80,11 @@ Request = {
 
     local req = {
       ("%s %s RTSP/1.0"):format(self.method, self.url),
-      ("CSeq: %d"):format(self.cseq),
-      table.unpack(self.headers),
-      ""
+      ("CSeq: %d"):format(self.cseq)
     }
+    table.move(self.headers, 1, #self.headers, #req + 1, req)
+    table.insert(req, "")
+    table.insert(req, "")
     return table.concat(req, "\r\n")
   end,
 }