ISSUE: Heap corruption occurs and hence mysql server
       terminates abnormally in String variable destructor
       when ZEROFILL is used for a column.
       Though the abnormal termination is observed in the
       String destructor, heap corruption occurs at earlier
       stage when function Field_num::prepend_zeros() is called.
       This function, prepends zeros to the actual data and
       works on entire field length. Since the allocated memory
       could be less than the field length, heap corruption occurs.
       Later, when String destructor tries to free heap, the server
       terminates abnormally since the heap is corrupt.



SOLUTION: In Field_num::prepend_zeros() function, if allocated memory
          is less than the field length, re-allocate memory enough to
          hold field length size data.