pgsql: Guard against incoming rowcount estimate of NaN in cost_mergejoi

Guard against incoming rowcount estimate of NaN in cost_mergejoin().

Although rowcount estimates really ought not be NaN, a bug elsewhere
could perhaps result in that, and that would cause Assert failure in
cost_mergejoin, which I believe to be the explanation for bug #5977 from
Anton Kuznetsov.  Seems like a good idea to expend a couple more cycles
to prevent that, even though the real bug is elsewhere.  Not back-patching,
though, because we don't encourage running production systems with
Asserts on.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/72826fb362c4aada6d2431df0b706df448806c02

Modified Files
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src/backend/optimizer/path/costsize.c |    6 +++---
1 files changed, 3 insertions(+), 3 deletions(-)