avio_skip() does not return EOF if offset is large

[Guangyu Sun]

Summary of the bug:
When calling

int64_t avio_skip(AVIOContext *s, int64_t offset)

on an avio-opened file, EOF is returned if skipped beyond the end of the file.
But if offset is greater than the internal buffer size (default is IO_BUFFER_SIZE 32768), EOF will not be returned.

How to reproduce:
Write a program with libavformat:

#include <libavformat/avio.h>
int main(int argc, char *argv[])
{
    if (argc < 3)
        return 1;
    int64_t offset = atoll(argv[1]);
    AVIOContext *avio_ctx = NULL;
    avio_open2(&avio_ctx, argv[2], AVIO_FLAG_READ, NULL, NULL);
    int64_t res = avio_skip(avio_ctx, offset);
    printf("res %"PRId64" %s\n", res, av_err2str(res));
    return 0;
}

Compile the program to a binary with name "test" and run:

# seek with an 8-byte file
$ echo 12345678 > testfile
$ ls -l testfile
-rw-r--r-- 1 gsun staff 8 Mar 29 09:06 testfile
$ ./test 8 testfile
res 8 Error number 8 occurred
$ ./test 9 testfile
res -541478725 End of file
$ ./test 32768 testfile
res -541478725 End of file
$ ./test 32769 testfile
res 32769 Error number 32769 occurred