Correct comparisons between size_t and ssize_t

Correct comparisons between size_t and ssize_t

* src/sfnt.c (sfnt_read_table_directory, sfnt_read_cmap_format_0)
(sfnt_read_cmap_format_2, sfnt_read_cmap_format_4)
(sfnt_read_cmap_format_6, sfnt_read_cmap_format_8)
(sfnt_read_cmap_format_12, sfnt_read_cmap_format_14)
(sfnt_read_cmap_table_1, sfnt_read_cmap_table)
(sfnt_read_head_table, sfnt_read_hhea_table, sfnt_read_maxp_table)
(sfnt_read_glyf_table, sfnt_read_hmtx_table, sfnt_read_name_table)
(sfnt_read_meta_table, sfnt_read_ttc_header)
(sfnt_read_default_uvs_table, sfnt_read_nondefault_uvs_table)
(sfnt_read_fvar_table, sfnt_read_gvar_table, sfnt_read_avar_table)
(sfnt_read_cvar_table, sfnt_read_OS_2_table):

* src/sfntfont.c (sfnt_enum_font): Revise code written with only
signed `long' in mind to properly compare size_t values with
ssize_t.