pgsql: Fix line_construct_pm() for the case of "infinite" (DBL_MAX) slo

Fix line_construct_pm() for the case of "infinite" (DBL_MAX) slope.

This code was just plain wrong: what you got was not a line through the
given point but a line almost indistinguishable from the Y-axis, although
not truly vertical. The only caller that tries to use this function with
m == DBL_MAX is dist_ps_internal for the case where the lseg is horizontal;
it would end up producing the distance from the given point to the place
where the lseg's line crosses the Y-axis. That function is used by other
operators too, so there are several operators that could compute wrong
distances from a line segment to something else. Per bug #5745 from
jindiax.

Back-patch to all supported branches.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=5abb72f9af64339ed97eb991b0bd3456bd651f8e

Modified Files
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src/backend/utils/adt/geo_ops.c | 15 +++++++++++----
1 files changed, 11 insertions(+), 4 deletions(-)