### What it does
Checks for calls to [`splitn`]
(https://doc.rust-lang.org/std/primitive.str.html#method.splitn) and
related functions with either zero or one splits.

### Why is this bad?
These calls don't actually split the value and are
likely to be intended as a different number.

### Example
```
for x in s.splitn(1, ":") {
    // ..
}
```

Use instead:
```
for x in s.splitn(2, ":") {
    // ..
}
```