patch 9.0.0189: invalid memory access for text prop without highlight

Problem:    Invalid memory access for text prop without highlight.
Solution:   Check for a valid highlight ID.

src/highlight.c

@@ -3504,6 +3504,7 @@ syn_unadd_group(void)
 
 /*
  * Translate a group ID to highlight attributes.
+ * "hl_id" must be valid: > 0, caller must check.
  */
     int
 syn_id2attr(int hl_id)