CURSOR + SELECT INTO ..

ANALYSIS:
=========
While executing a stored program, a debug assert is
triggered for a SELECT query if the derived table resolving
failed for the previous sub-statement of the stored
program.
In non-debug build, execution of the statement in such a
scenario will yield unexpected result.

TABLE has a bitmap read_set. A bit in this bitmap is set
for a field of a table used in the query. But this bitmap
is not set (for outer-most query block) while resolving
derived tables. A flag THD::derived_tables_processing is
used to indicate the derived table processing.

In case of a stored program, if the derived table resolving
fails then the flag THD::derived_tables_processing is not
reset for the stored program statement. Due to this,
read_set of TABLE is not set during the execution of next
stored program statement. Hence an assert condition that
checks for the state of read_set fails.

In non-debug builds, the second SELECT query execution
completes but returns wrong result set.

FIX:
====
THD::derived_tables_processing flag is now reset before
execution of each sub-statement in the stored program.

Change-Id: I84bc5d8b509d341ca7c58d12241992672135b6bf