Make String.spit_n work with n > len

To make things consistent with drop/take

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/stdune/string.ml

@@ -249,11 +249,7 @@ let drop s n =
 
 let split_n s n =
   let len = length s in
-  if n > len then
-    Exn.code_error "String.split_n"
-      [ "s", Sexp.Atom s
-      ; "n", Sexp.Atom (string_of_int n)
-      ];
+  let n = min n len in
   ( sub s ~pos:0 ~len:n
   , sub s ~pos:n ~len:(len - n)
   )

test/unit-tests/string.mlt

@@ -57,11 +57,7 @@ String.split_n "foobar" 3;;
 
 String.split_n "foobar" 10;;
 [%%expect{|
-Exception:
-Code_error
- (List
-   [Atom "String.split_n"; List [Atom "s"; Atom "foobar"];
-    List [Atom "n"; Atom "10"]]).
+- : string * string = ("foobar", "")
 |}]
 
 String.split_n "foobar" 0;;
@@ -81,9 +77,5 @@ String.split_n "" 0;;
 
 String.split_n "" 10;;
 [%%expect{|
-Exception:
-Code_error
- (List
-   [Atom "String.split_n"; List [Atom "s"; Atom ""];
-    List [Atom "n"; Atom "10"]]).
+- : string * string = ("", "")
 |}]