Fix optional element detecting

mirai-core-qqandroid/src/commonMain/kotlin/net/mamoe/mirai/qqandroid/io/serialization/jce/JceDecoder.kt

@@ -231,11 +231,12 @@ internal class JceDecoder(
 
     override fun decodeSequentially(): Boolean = false
     override fun decodeElementIndex(descriptor: SerialDescriptor): Int {
-        val jceHead = jce.currentHeadOrNull ?: return CompositeDecoder.READ_DONE
+        var jceHead = jce.currentHeadOrNull ?: return CompositeDecoder.READ_DONE
         if (jceHead.type == Jce.STRUCT_END) {
             return CompositeDecoder.READ_DONE
         }
 
+        while (!jce.input.endOfInput){
             repeat(descriptor.elementsCount) {
                 val tag = descriptor.getJceTagId(it)
                 if (tag == jceHead.tag) {
@@ -243,6 +244,13 @@ internal class JceDecoder(
                 }
             }
 
+            jce.skipField(jceHead.type)
+            if (!jce.prepareNextHead()) {
+                break
+            }
+            jceHead = jce.currentHead
+        }
+
         return CompositeDecoder.READ_DONE // optional support
     }