[base, sfnt] Better checks for invalid cmaps (1/2). * src/base/ftobjs.c (FT_Get_Char_Index): Don't return out-of-bounds glyph indices. (FT_Get_First_Char): Updated. * src/sfnt/ttcmap.c (tt_cmap6_char_next): Don't return character codes greater than 0xFFFF. (tt_cmap8_char_index): Avoid integer overflow in computation of glyph index. (tt_cmap8_char_next): Avoid integer overflows in computation of both next character code and glyph index. (tt_cmap10_char_index): Fix unsigned integer logic. (tt_cmap10_char_next): Avoid integer overflow in computation of next character code. (tt_cmap12_next): Avoid integer overflows in computation of both next character code and glyph index. (tt_cmap12_char_map_binary): Ditto. (tt_cmap12_char_next): Simplify. (tt_cmap13_char_map_binary): Avoid integer overflow in computation of next character code. (tt_cmap13_char_next): Simplify.