BufferReadUntilBlock should release buf when no bytes read

This was causing allocations every time there was a non-blocking read
with nothing to read.

https://github.com/jackc/pgx/issues/1481

internal/nbconn/nbconn.go

@@ -317,6 +317,8 @@ func (c *NetConn) BufferReadUntilBlock() error {
 		if n > 0 {
 			buf = buf[:n]
 			c.readQueue.pushBack(buf)
+		} else if n == 0 {
+			iobufpool.Put(buf)
 		}
 
 		if err != nil {