pgsql: Disallow NaN as a value for floating-point GUCs.

Disallow NaN as a value for floating-point GUCs.

None of the code that uses GUC values is really prepared for them to
hold NaN, but parse_real() didn't have any defense against accepting
such a value.  Treat it the same as a syntax error.

I haven't attempted to analyze the exact consequences of setting any
of the float GUCs to NaN, but since they're quite unlikely to be good,
this seems like a back-patchable bug fix.

Note: we don't need an explicit test for +-Infinity because those will
be rejected by existing range checks.  I added a regression test for
that in HEAD, but not older branches because the spelling of the value
in the error message will be platform-dependent in branches where we
don't always use port/snprintf.c.

Discussion: https://postgr.es/m/1798.1552165479@sss.pgh.pa.us

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e04bb261633dd76d6dc2e250c92f33330ec2891e

Modified Files
--------------
src/backend/utils/misc/guc.c      | 4 ++++
src/test/regress/expected/guc.out | 5 +++++
src/test/regress/sql/guc.sql      | 4 ++++
3 files changed, 13 insertions(+)