Fix segfault while doing reverse K on NULL queue

[yanjiew1]

In do_reverseK, we should check whether current is NULL before dereferencing current. We should not dereference it if it is NULL.

[eecheng87]

Hi @yanjiew1 ,

There actually are several do_.* functions lack of null-pointer detection, can you also consolidate them in this commit?
Also, can you make the detection be consistent to other functions? For example:

static bool do_reverse(int argc, char *argv[])
{
...

    if (!current || !current->q)
        report(3, "Warning: Calling reverse on null queue");
...
}

Thanks.

[yanjiew1]

Pull request #125 has already fixed this problem.