CVE-2017-13011/Properly check for buffer overflow in bittok2str_internal().

Also, make the buffer bigger.

This fixes a buffer overflow discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s), modified
so the capture file won't be rejected as an invalid capture.
safeputchar(netdissect_options *ndo,
            const u_char c)
{
	ND_PRINT((ndo, (c < 0x80 && ND_ISPRINT(c)) ? "%c" : "\\0x%02x", c));
}

safeputchar(netdissect_options *ndo,
            const u_char c)
{
	ND_PRINT((ndo, (c < 0x80 && ND_ISPRINT(c)) ? "%c" : "\\0x%02x", c));
}
