bsdiff: fix off-by-one error

[mhorne]

Summary

The program reads oldsize bytes from oldfile, and proceeds to initialize
a suffix array of oldsize elements using divsufsort(). As per the
function's API [1], array indices 0 through n-1 are initialized.

Later, search() is called, but with index bounds [0, n]. Depending on
the contents of the malloc'd buffer, accessing this uninitialized index
at the end of can result in a segmentation fault. Fix this by passing
oldsize-1 to search(), making the search bounds [0, n-1].

[1] https://github.com/y-256/libdivsufsort

Test Plan

This fixes the segfault I experienced at the end of building an update, on arm64.

[cperciva]

Looks good to me. This is a bug introduced in r303285; the original qsufsort sorted oldsize+1 strings including an implicit empty string, but it looks like the divsufsort which replaced it doesn't do this.