fix possible copy of null buffer and validation of unitialized header

fix possible copy of null buffer and validation of unitialized header

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

 fwtool.c:257:2: warning: Null pointer passed as an argument to a 'nonnull' parameter
        memcpy(dest, dbuf->cur + dbuf->cur_len - cur_len, cur_len);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 fwtool.c:275:20: warning: The left operand of '!=' is a garbage value
         if (hdr->version != 0)
             ~~~~~~~~~~~~ ^

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