Issue
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This problem occurs when varchar columns are used in a
internal temporary table. The type of the field is set
incorrectly to the generic FIELD_NORMAL type. This in turn
results in an inaccurate calculation of the record length.
Valgrind issues will occur since initialization has not
happend for some bytes.

Fix
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While creating the temporary table, the type of the field
needs to be to set FIELD_VARCHAR. This will allow myisam
to calculate the record length accurately.

This fix is a backport of BUG#13350136.