fix issue #70, bug introduced in recent memory profiler improvements

(cherry picked from commit 3ae6ade16a2cd3c3dec25b56c90c67330293284a)

Author: oranagra

rdbtools/memprofiler.py

@@ -78,6 +78,8 @@ def __init__(self, out):
                                                  "size_in_bytes", "encoding", "num_elements", "len_largest_element"))
 
     def next_record(self, record) :
+        if record.key is None:
+            return  # some records are not keys (e.g. dict)
         self._out.write("%d,%s,%s,%d,%s,%d,%d\n" % (record.database, record.type, encode_key(record.key), 
                                                  record.bytes, record.encoding, record.size, record.len_largest_element))