scripts/kallsyms.c: fix potential segfault

Description:
This bug hardly appears during real kernel compiling,
 because the vmlinux symbols table is huge.

But we can still catch it under strict condition , as follows.
   $ echo "c101b97b T do_fork" | ./scripts/kallsyms --all-symbols
   #include <asm/types.h>
   ......
   ......
   .globl kallsyms_token_table
           ALGN
   kallsyms_token_table:
   Segmentation fault (core dumped)
   $

If symbols table is small, all entries in token_profit[0x10000] may
decrease to 0 after several calls of compress_symbols() in optimize_result().
In that case, find_best_token() always return 0 and
best_table[i] is set to "\0\0" and best_table_len[i] is set to 2.

As a result, expand_symbol(best_table[0]="\0\0", best_table_len[0]=2, buf)
in write_src() will run in infinite recursion until stack overflows,
causing segfault.

This patch checks the find_best_token() return value. If all entries in
token_profit[0x10000] become 0 according to return value, it breaks the loop...