refactor(rust): Fix fencepost debug assertion in splitfields

For some slice `v` of length `n`, it is UB to call `get_unchecked`
with a range starting at `n+1`. Previously, we asserted that `pos <=
n`, which is the requirement for `v.get_unchecked(..pos)`, but not
`v.get_unchecked(pos+1..)` which has the tighter requirement `pos <
n`.

crates/polars-io/src/csv/read/splitfields.rs

@@ -302,7 +302,7 @@ mod inner {
             };
 
             unsafe {
-                debug_assert!(pos <= self.v.len());
+                debug_assert!(pos < self.v.len());
                 // SAFETY:
                 // we are in bounds
                 let ret = Some((self.v.get_unchecked(..pos), needs_escaping));