Field_varstring::store
      
The code that temporary saved the bitmaps of the read set and the write set so that
it can set it to all columns for debug purposes was not expecting that the
table->read_set and table->write_set can be the same. And was always saving both in 
sequence.
As a result the original value was never restored.
Fixed by saving & restoring the original value only once if the two sets are the
same (in a special set of functions).