LibJS: Add missing assert to TypedArray.prototype.indexOf

Author: https://github.com/jamierocks
Commit: 70a4e4bd67
Pull-request: https://github.com/SerenityOS/serenity/pull/16117
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Runtime/TypedArrayPrototype.cpp

@@ -681,7 +681,9 @@ JS_DEFINE_NATIVE_FUNCTION(TypedArrayPrototype::index_of)
     // 5. Let n be ? ToIntegerOrInfinity(fromIndex).
     auto n = TRY(vm.argument(1).to_integer_or_infinity(vm));
 
-    // FIXME: 6. Assert: If fromIndex is undefined, then n is 0.
+    // 6. Assert: If fromIndex is undefined, then n is 0.
+    if (vm.argument(1).is_undefined())
+        VERIFY(n == 0);
 
     auto value_n = Value(n);
     // 7. If n is +∞, return -1𝔽.