x.json2.decoder2: fix array decoding in struct is field, when array is initialized (#24422)

vlib/x/json2/decoder2/decode.v

@@ -618,6 +618,9 @@ fn (mut decoder Decoder) decode_value[T](mut val T) ! {
 		decoder.decode_map(mut val)!
 		return
 	} $else $if T.unaliased_typ is $array {
+		unsafe {
+			val.len = 0
+		}
 		decoder.decode_array(mut val)!
 		// return to avoid the next increment of the current node
 		// this is because the current node is already incremented in the decode_array function