Partial fix for dropped columns blowing up replication when replaying binlog with past events

Mitigates following errror:
  File "/usr/local/lib/python2.7/dist-packages/pymysqlreplication/binlogstream.py", line 262, in fetchone
    self.__freeze_schema)
  File "/usr/local/lib/python2.7/dist-packages/pymysqlreplication/packet.py", line 98, in __init__
    freeze_schema = freeze_schema)
  File "/usr/local/lib/python2.7/dist-packages/pymysqlreplication/row_event.py", line 550, in __init__
    column_schema = self.column_schemas[i]
IndexError: list index out of range

This is just a superficial patch. It does not address deeper issue at hand, which is that pymysqlreplication
has no good way of handling RowEvents containing columns which have been dropped prior to pymsqlreplication
start. The problem is always relying on current version of information_schema even though dealing with events
that might be several versions in the past.

Author: Bartek Ogryczak

pymysqlreplication/row_event.py

@@ -547,7 +547,20 @@ def __init__(self, from_packet, event_size, table_map, ctl_connection, **kwargs)
             self.packet.read_length_coded_binary()
             for i in range(0, len(column_types)):
                 column_type = column_types[i]
-                column_schema = self.column_schemas[i]
+                try:
+                    column_schema = self.column_schemas[i]
+                except IndexError:
+                    # this a dirty hack to prevent row evens containing columns which have been dropped prior
+                    # to pymysqlreplication start, but replayed from binlog from blowing up the service.
+                    # TODO: this does not address the issue if the column other than the last one is dropped
+                    column_schema = {
+                        'COLUMN_NAME': '__dropped_col_{i}__'.format(i=i),
+                        'COLLATION_NAME': None,
+                        'CHARACTER_SET_NAME': None,
+                        'COLUMN_COMMENT': None,
+                        'COLUMN_TYPE': 'BLOB',  # we don't know what it is, so let's not do anything with it.
+                        'COLUMN_KEY': '',
+                    }
                 col = Column(byte2int(column_type), column_schema, from_packet)
                 self.columns.append(col)