make split_at_mut similar to split_at and use the checked function

Author: KGrewal1

library/core/src/str/mod.rs

@@ -678,12 +678,9 @@ impl str {
     #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
     pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
-        // is_char_boundary checks that the index is in [0, .len()]
-        if self.is_char_boundary(mid) {
-            // SAFETY: just checked that `mid` is on a char boundary.
-            unsafe { self.split_at_mut_unchecked(mid) }
-        } else {
-            slice_error_fail(self, 0, mid)
+        match self.split_at_mut_checked(mid) {
+            None => slice_error_fail(self, 0, mid),
+            Some(pair) => pair,
         }
     }