ffprobe: Fix null pointer dereference with color primaries

Found-by: AD-lab of venustech
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static const char *json_escape_str(AVBPrint *dst, const char *src, void *log_ctx)
{
    static const char json_escape[] = {'"', '\\', '\b', '\f', '\n', '\r', '\t', 0};
    static const char json_subst[]  = {'"', '\\',  'b',  'f',  'n',  'r',  't', 0};
    const char *p;

    for (p = src; *p; p++) {
        char *s = strchr(json_escape, *p);
        if (s) {
            av_bprint_chars(dst, '\\', 1);
            av_bprint_chars(dst, json_subst[s - json_escape], 1);
        } else if ((unsigned char)*p < 32) {
            av_bprintf(dst, "\\u00%02x", *p & 0xff);
        } else {
            av_bprint_chars(dst, *p, 1);
        }
    }
    return dst->str;
}

static const char *json_escape_str(AVBPrint *dst, const char *src, void *log_ctx)
{
    static const char json_escape[] = {'"', '\\', '\b', '\f', '\n', '\r', '\t', 0};
    static const char json_subst[]  = {'"', '\\',  'b',  'f',  'n',  'r',  't', 0};
    const char *p;

    for (p = src; *p; p++) {
        char *s = strchr(json_escape, *p);
        if (s) {
            av_bprint_chars(dst, '\\', 1);
            av_bprint_chars(dst, json_subst[s - json_escape], 1);
        } else if ((unsigned char)*p < 32) {
            av_bprintf(dst, "\\u00%02x", *p & 0xff);
        } else {
            av_bprint_chars(dst, *p, 1);
        }
    }
    return dst->str;
}
