patch 8.0.0672: third item of synconcealed() changes too often

Problem:    Third item of synconcealed() changes too often. (Dominique Pelle)
Solution:   Reset the sequence number at the start of each line.
diff --git a/src/syntax.c b/src/syntax.c
index 45d9bda..da2c876 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -1061,6 +1061,7 @@
 
     next_match_idx = -1;
     ++current_line_id;
+    next_seqnr = 1;
 }
 
 /*
@@ -1857,6 +1858,7 @@
 #endif
 #ifdef FEAT_CONCEAL
 	current_flags = 0;
+	current_seqnr = 0;
 #endif
 	return 0;
     }
@@ -2346,6 +2348,7 @@
 #endif
 #ifdef FEAT_CONCEAL
     current_flags = 0;
+    current_seqnr = 0;
 #endif
     if (cur_si != NULL)
     {