Fix an incorrect assertion in op_pcm_seek_page().
When we were checking the current file offset to see if we should use it as the starting bisection point, we assumed that offset was larger than the start of the data range for that link (and consequently, inside the bisection range). If there is a random page earlier in the file that happens to use the same serial number as a link we identified later in the file at file open time, and we had stopped reading there before the seek, then this assumption might not be true. Ironically, it was not the case that contained the assertion that had trouble with such an offset. It would fail the check that we were cutting off more than half the range, since we were actually cutting off a negative amount, and fall back to the midpoint of the link as the first bisection point. However, the case below that (where the target comes after the current timestamp), we might have erroneously cut off the entire range (setting end to offset, which was less than begin), causing the seek to immediately fail. Instead, validate the curent offset against both ends of the link before attempting to use it as the initial bisection point. Thanks to Felicia Lim for the report. Fixes #2331