fix possible garbage in unitialized char* struct members

fix possible garbage in unitialized char* struct members

scan-build from clang version 9 has reported following issues:

 crc32.h:44:32: warning: The right operand of '^' is a garbage value
                val = crc_table[(uint8_t)val ^ *(uint8_t*)buf] ^ (val >> 8);
                                             ^ ~~~~~~~~~~~~~~

cppcheck version 1.89 has reported following issues:

 fwtool.c:260:9: error: Uninitialized variable: dest [uninitvar]
  memcpy(dest, dbuf->cur + dbuf->cur_len - cur_len, cur_len);
         ^
 fwtool.c:333:27: note: Calling function 'extract_tail', 2nd argument '&tr' value is <Uninit>
   if (extract_tail(&dbuf, &tr, sizeof(tr))) {
                           ^

Signed-off-by: Petr Štetiar <ynezz@true.cz>