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

src/backend/storage/buffer/bufmgr.c

@@ -514,7 +514,7 @@ ComputeIoConcurrency(int io_concurrency, double *target)
 	*target = new_prefetch_pages;
 
 	/* This range check shouldn't fail, but let's be paranoid */
-	return (new_prefetch_pages > 0.0 && new_prefetch_pages < (double) INT_MAX);
+	return (new_prefetch_pages >= 0.0 && new_prefetch_pages < (double) INT_MAX);
 }
 
 /*