Check for index-out-of-bound in elementAt function (KT-30051)

libraries/stdlib/test/text/StringTest.kt

@@ -1460,4 +1460,14 @@ ${"    "}
         assertEquals("  ABC\n   \n  123", "ABC\n   \n123".prependIndent("  "))
         assertEquals("  ", "".prependIndent("  "))
     }
+
+    @Test
+    fun elementAt() {
+        expect('a') { "a c".elementAt(0) }
+        expect(' ') { "a c".elementAt(1) }
+        expect('c') { "a c".elementAt(2) }
+
+        assertFailsWith<IndexOutOfBoundsException> { "".elementAt(0) }
+        assertFailsWith<IndexOutOfBoundsException> { "a c".elementAt(-1) }
+    }
 }