fix error with message reports when custom checker uses old-style messages

when register_messages is called, it allows messages that are in the form
of 2-tuples, which means a symbol is not necessary. however, when doing
report_message_stats is called, it does not handle the case where there is
no symbol.

solution: when collecting message, use the message id as the key if the
symbol is None.

e.g.
```
Messages
--------

+------------------+------------+
|message id        |occurrences |
+==================+============+
|missing-docstring |6           |
+------------------+------------+
|W9901             |4           |
+------------------+------------+
|invalid-name      |1           |
+------------------+------------+
```

1 files changed, 4 insertions, 2 deletions

diff --git a/utils.py b/utils.py
index 59e22a5..a09be51 100644
--- a/utils.py
+++ b/utils.py
@@ -400,6 +400,8 @@ class MessagesHandlerMixIn(object):
         """
         msg_info = self.check_message_id(msg_descr)
         msgid = msg_info.msgid
+        # backward compatibility, message may not have a symbol
+        symbol = msg_info.symbol or msgid
         # Fatal messages and reports are special, the node/scope distinction
         # does not apply to them.
         if msgid[0] not in _SCOPE_EXEMPT:
@@ -427,9 +429,9 @@ class MessagesHandlerMixIn(object):
         self.stats[msg_cat] += 1
         self.stats['by_module'][self.current_name][msg_cat] += 1
         try:
-            self.stats['by_msg'][msg_info.symbol] += 1
+            self.stats['by_msg'][symbol] += 1
         except KeyError:
-            self.stats['by_msg'][msg_info.symbol] = 1
+            self.stats['by_msg'][symbol] = 1
         # expand message ?
         msg = msg_info.msg
         if args: