fix: prevent index out-of-range panic in hasCommonSubstring

[edznux-dd]

👋, I was looking a some of my app and discovered a small bugs in the library.
So here's a fix and some description of the issue!

I took the liberty to add a fuzz test as I don't think you have any fuzz test in here yet. I've run it for a bit after the fix and didn't seem to find any other bugs :)

Description

Distance parses hash strings by splitting on : but imposes no length cap on the body segments. hasCommonSubstring allocates a hashes slice of fixed size spamSumLength - (rollingWindow - 1) = 64 - 6 = 58, then writes into it at index i-(rollingWindow-1) while looping for i = rollingWindow-1; i < s1Len; i++. When the hash body is 65 or more characters long the index reaches 58, which is out of bounds, and the runtime raises a fatal panic.

Crash Input

I've added a regression test for this case, but for convenience:

s1 := "3:" + strings.Repeat("0", 65) + ":"
s2 := "3:0000000:"
ssdeep.Distance(s1, s2) // panic: runtime error: index out of range [58] with length 58

Root Cause

In score.go:104:

// Vulnerable:
func hasCommonSubstring(s1, s2 string) bool {
    // ...
    hashes := make([]uint32, (spamSumLength - (rollingWindow - 1))) // length = 58
    // ...
    for i = rollingWindow - 1; i < s1Len; i++ {
        state.rollHash(s1[i])
        hashes[i-(rollingWindow-1)] = state.rollSum() // panics when i-(rollingWindow-1) >= 58
    }
    // ...
}

[glaslos]

@edznux-dd can you rebase? I updated main to go1.18 to support testing.F