pgsql: Reduce an unnecessary O(N^3) loop in lexer.

Reduce an unnecessary O(N^3) loop in lexer.

The lexer's handling of operators contained an O(N^3) hazard when
dealing with long strings of + or - characters; it seems hard to
prevent this case from being O(N^2), but the additional N multiplier
was not needed.

Backpatch all the way since this has been there since 7.x, and it
presents at least a mild hazard in that trying to do Bind, PREPARE or
EXPLAIN on a hostile query could take excessive time (without
honouring cancels or timeouts) even if the query was never executed.

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ad871a9d78841592938a0b24cc961fd1d4185607

Modified Files
--------------
src/backend/parser/scan.l | 29 +++++++++++++++++++++--------
src/bin/psql/psqlscan.l | 29 +++++++++++++++++++++--------
src/interfaces/ecpg/preproc/pgc.l | 29 +++++++++++++++++++++--------
3 files changed, 63 insertions(+), 24 deletions(-)