Fix short-circuit test when seeking in short files

When a file is very, very short (i.e., only one packet) and uses end-trimming, the apparent granule position preceding the first sample in the first packet can underflow. We were computing this value by subtracting the packet duration from the computed per-packet granule position and expecting this computation to always succeed. Because it could fail in the presence of end-trimming on the first packet (ironically, exactly the situation where the short-circuit is helpful), it would leave the value uninitialized, and then use it in a comparison, which is undefined behavior. The correct solution is to check for failure and force the previous page's granule position to 0 in this case.