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
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