jpeg: Fix another possible buffer overrun

Found via the clang libfuzzer
void pdf_clear_err(struct pdf_doc *pdf)
{
    if (!pdf)
        return;
    pdf->errstr[0] = '\0';
    pdf->errval = 0;
}

void pdf_clear_err(struct pdf_doc *pdf)
{
    if (!pdf)
        return;
    pdf->errstr[0] = '\0';
    pdf->errval = 0;
}
