ArrayIndexOutOfBoundsException in appendStringSlowPath when serializing escaped characters

[JBeet]

Describe the bug
An ArrayIndexOutOfBoundsException is thrown in appendStringSlowPath when serializing strings that contain many characters that wil be escaped in the generated JSON. It looks like this is caused by an insufficient ensureAdditionalCapacity(string.length + 2) in appendQuoted

To Reproduce

import kotlinx.serialization.Serializable
import kotlinx.serialization.encodeToString
import kotlinx.serialization.json.Json

fun main() {
    println(Json.encodeToString(SerializationTest("\"".repeat(100))))
}

@Serializable
data class SerializationTest(val s: String)

Expected behavior
String is successfully serialized as JSON, without error.

Environment

• Kotlin version: 1.5.0
• Library version: 1.2.0
• Kotlin platforms: JVM
• Gradle version: 6.8