pgsql: Use mul_size when multiplying by the number of parallel workers.

Use mul_size when multiplying by the number of parallel workers.

That way, if the result overflows size_t, you'll get an error instead
of undefined behavior, which seems like a plus.  This also has the
effect of casting the number of workers from int to Size, which is
better because it's harder to overflow int than size_t.

Dilip Kumar reported this issue and provided a patch upon which this
patch is based, but his version did use mul_size.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/06bd458cb812623c3f1fdd55216c4c08b06a8447

Modified Files
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src/backend/access/transam/parallel.c |  6 ++++--
src/backend/executor/execParallel.c   | 18 +++++++++++-------
2 files changed, 15 insertions(+), 9 deletions(-)