pgsql: Ensure maxlen is at leat 1 in dict_int

Ensure maxlen is at leat 1 in dict_int

The dict_int text search dictionary template accepts maxlen parameter,
which is then used to cap the length of input strings. The value was
not properly checked, and the code simply does

txt[d->maxlen] = '\0';

to insert a terminator, leading to segfaults with negative values.

This commit simply rejects values less than 1. The issue was there since
dct_int was introduced in 9.3, so backpatch all the way back to 9.4
which is the oldest supported version.

Reported-by: cili
Discussion: https://postgr.es/m/16144-a36a5bef7657047d@postgresql.org
Backpatch-through: 9.4

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b5273943679d22f58f1e1e269ad75e791172f557

Modified Files
--------------
contrib/dict_int/dict_int.c | 5 +++++
contrib/dict_int/expected/dict_int.out | 2 ++
contrib/dict_int/sql/dict_int.sql | 2 ++
3 files changed, 9 insertions(+)