Fix syntax crash from null filter

When a filter or validate function throws null,
the global event loop in loop.js unsafely tries
to access its properties, crashing couchjs.
This adds safe null checks to the global handler
to prevent complete denial of service.

Author: vamsi

share/server/loop.js

@@ -135,6 +135,10 @@ var Loop = function() {
     "rereduce" : Views.rereduce
   };
   function handleError(e) {
+    if (e == null) {
+      respond(["error", "unnamed_error", String(e)]);
+      return;
+    }
     var type = e[0];
     if (type == "fatal") {
       e[0] = "error"; // we tell the client it was a fatal error by dying

share/server/validate.js

@@ -16,7 +16,7 @@ var Validate = {
       fun.apply(ddoc, args);
       respond(1);
     } catch (error) {
-      if (error.name && error.stack) {
+      if (error && error.name && error.stack) {
         throw error;
       }
       respond(error);