shaper: fix reallocation

Update the variable that tracks the allocated size. This potentially
improves performance and avoid some side effects, which lead to
undefined behavior in some cases.

Fixes fuzzer test case id:000051,sig:11,sync:fuzzer3,src:004221.
get_v_kerning(hb_font_t *font, void *font_data, hb_codepoint_t first,
                 hb_codepoint_t second, void *user_data)
{
    return 0;
}

get_v_kerning(hb_font_t *font, void *font_data, hb_codepoint_t first,
                 hb_codepoint_t second, void *user_data)
{
    return 0;
}
