pgsql: Fix contrib/seg to be more wary of long input numbers.

Fix contrib/seg to be more wary of long input numbers.

seg stores the number of significant digits in an input number
in a "char" field. If char is signed, and the input is more than
127 digits long, the count can read out as negative causing
seg_out() to print garbage (or, if you're really unlucky,
even crash).

To fix, clamp the digit count to be not more than FLT_DIG.
(In theory this loses some information about what the original
input was, but it doesn't seem like useful information; it would
not survive dump/restore in any case.)

Also, in case there are stored values of the seg type containing
bad data, add a clamp in seg_out's restore() subroutine.

Per bug #17725 from Robins Tharakan. It's been like this
forever, so back-patch to all supported branches.

Discussion: https://postgr.es/m/17725-0a09313b67fbe86e@postgresql.org

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d35f1d485c392ce058382794742ce246e47643ec

Modified Files
--------------
contrib/seg/expected/seg.out | 7 +++++++
contrib/seg/seg.c | 8 ++++++--
contrib/seg/segparse.y | 22 +++++++++++++++++-----
contrib/seg/sql/seg.sql | 3 +++
4 files changed, 33 insertions(+), 7 deletions(-)