[YouTube] Access first element if array size is one

Fixes a regression, where if the challenge data array size was one, the second element
would be accessed, leading to a crash.
This was introduced when porting the challenge parsing from JS to
Kotlin.

Ref: 53b599b042

app/src/main/java/org/schabi/newpipe/util/potoken/JavaScriptUtil.kt

@@ -17,7 +17,7 @@ fun parseChallengeData(rawChallengeData: String): String {
         val descrambled = descramble(scrambled.getString(1))
         JsonParser.array().from(descrambled)
     } else {
-        scrambled.getArray(1)
+        scrambled.getArray(0)
     }
 
     val messageId = challengeData.getString(0)