Fix checkptr in AddString64

[somnusfish]

Related issue : #12

Root cause:

According to https://pkg.go.dev/unsafe#Pointer, Unlike in C, it is not valid to advance a pointer just beyond the end of its original allocation:

// INVALID: end points outside allocated space.
var s thing
end = unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Sizeof(s))

// INVALID: end points outside allocated space.
b := make([]byte, n)
end = unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(n))

The original logic hit the edge case when the length of the string could be divided by 8.

Solution

Handle the edge case using if condition, please refer the code for details.

Testing

Local fuzz test.

[yerden]

hi @somnusfish thanks for a fix. I managed to slightly improve readability if you don't mind. You could take over my code in #14 or @achille-roussel may green light it as it is.

[somnusfish]

Hi @yerden , thanks for the feedback. I took over your code since it is much faster and better readable.