Fix range check for effective_io_concurrency

Fix range check for effective_io_concurrency

Commit 1aba62ec moved the range check of that option form guc.c into
bufmgr.c, but introduced a bug by changing a >= 0.0 to > 0.0, which made
the value 0 no longer accepted.  Put it back.

Reported by Jeff Janes, diagnosed by Tom Lane