a table name.
  
The problem was that fill_defined_view_parts() did not return
an error if a table is going to be altered. That happened if
the table was already in the table cache. In that case,
open_table() returned non-NULL value (valid TABLE-instance from
the cache).
  
The fix is to ensure that an error is thrown even if the table
is in the cache.

(This is a backport of the original patch for 5.1)