Description: Server may exit during SELECT on a MERGE table if
concurrent INSERT SELECT statements are executing in some
sequence.

Analysis: Optimizer creates a clone of MRG_INFO structure of
the merge table for index merge optimization during the
execution of SELECT statement on a MERGE table. Due to
concurrent INSERT SELECT statements, there arises a scenario
where the file_offset values maintained by the two MRG_INFO
structures becomes inconsistent. This in turn results in a
read from an incorrect position on the child table which
later results in the server crash.

Fix:- Table's main MRG_INFO structure is also updated with
the help of the function, 'info()' just after the clone for
MRG_INFO is created [inside 'ha_myisammrg::open()']. This
will make sure that all the values maintained by both the
structures will be in sync during the execution.