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
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b87037b373cf5605e6755e50c62348337a0d32c8

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(-)