libfyaml hangs indefinitely on `\x25\x22\xa6`

[MatejKafka]

If you compile the following .c file, with the current master of libfyaml:

#include "libfyaml.h"
#include <stdio.h>

int main(int argc, char* argv[]) {
    char* input = "\x25\x22\xa6";
    size_t len = 3;

    struct fy_document *fyd = NULL;
    fyd = fy_document_build_from_string(NULL, input, len);
    fy_document_destroy(fyd);

    return 0;
}

The parser hangs indefinitely in fy_scan_directive. There's a lot of similar hangs from a few hours of fuzzing with AFL++, I suspect the root cause will be the same or similar for most of them.

[pantoniou]

Fixed by 7f7be36f0305d2f8949f5a39c01f94e5babff615

Let me know if fuzzing comes up with more stuff like that.