The storage engine was not correctly handling the case in 
which rnd_pos is executed for a handler without a preceding
rnd_next or index read operation. As a result, an unitialized
file handle was sometimes being passed to the QMY_READ API.

The fix clears the rrnAssocHandle at the beginning of each
read operation and then checks to see whether it has been
set to a valid handle value before attempting to use it
in rnd_pos. If rrnAssocHandle has not been set by a previous
read operation, rnd_pos instead falls back to the use of the
currently active handle.

storage/ibmdb2i/ha_ibmdb2i.cc:
  Bug#44337 Select query using index merge fails with MCH3601
  
  - clear the rrnAssocHandle at the beginning of each
    read operation 
  - checks to see whether it has been set to a valid
    handle value before attempting to use it in rnd_pos