Fix format 2 cmap handling in sfnt.c

Fix format 2 cmap handling in sfnt.c

This code is untested as no font with a format 2 cmap could be found.

* src/sfnt.c (sfnt_lookup_glyph_2): Fix typos.  Assume single-byte
encodings use character codes 0, 1, ..., 255 rather than 0, 256, ...,
65280.

(cherry picked from commit 5c3d340e001187ad027bc0328f738938a2bc32c5)