pgsql: Make sampler_random_fract() actually obey its API contract.

Make sampler_random_fract() actually obey its API contract.

This function is documented to return a value in the range (0,1),
which is what its predecessor anl_random_fract() did. However, the
new version depends on pg_erand48() which returns a value in [0,1).
The possibility of returning zero creates hazards of division by zero
or trying to compute log(0) at some call sites, and it might well
break third-party modules using anl_random_fract() too. So let's
change it to never return zero. Spotted by Coverity.

Michael Paquier, cosmetically adjusted by me

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/cd7030ff085f5c378e837b392cb719cf23df9d0b

Modified Files
--------------
src/backend/utils/misc/sampling.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)