Fixed buffer overflow in printint

[HirbodBehnam]

Hi

There is a buffer overflow in printint function which can be triggered if numbers with more than 16 digits are written to the terminal. Numbers 18446744073709551615 and -9223372036854775808 both have 20 digits for instance. To this extent, I increased the buffer size to 20 characters which can fit base 10 of all 64 bit numbers (negative or positive).

I was also curious if this bug exists in x86 version of xv6. But apparently, the printint function used to accept an int instead of long long which was only 32 bits!