fix access to non existing backup index

Author: RCasatta

otsserver/rpc.py

@@ -71,22 +71,16 @@ def get_tip(self):
 
     def get_backup(self):
         chunk = self.path[len('/experimental/backup/'):]
+        result = b''
         try:
             chunk = int(chunk)
+            result = self.backup[chunk]
         except:
             self.send_response(404)
             self.send_header('Content-type', 'text/plain')
             self.end_headers()
             return
 
-        result = self.backup[chunk]
-
-        if result is None:
-            self.send_response(404)
-            self.send_header('Content-type', 'text/plain')
-            self.end_headers()
-            return
-
         self.send_response(200)
         self.send_header('Content-type', 'application/octet-stream')
         self.send_header('Cache-Control', 'public, max-age=31536000')