core: re-initialize branch 0 flags for each new message

Fixes branch 0 flags being "remembered" from previous script
invocations.

Reported-by: Juha Heinanen  jh at tutpro com
(cherry picked from commit c6dd54e8500b131a9dd53593afe009976ff3927f)

dset.c

@@ -231,6 +231,7 @@ void clear_branches(void)
 {
 	nr_branches = 0;
 	ruri_q = Q_UNSPECIFIED;
+	ruri_bflags = 0;
 }