pgsql: Some platforms set errno on pow(), exp() overflow, some do not,

Log Message:
-----------
Some platforms set errno on pow(), exp() overflow, some do not, so if
isinf(), fall through to our own infinity checks.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
float.c (r1.134 -> r1.135)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/float.c.diff?r1=1.134&r2=1.135)