Fix invalid array access in trgm_regexp.c.

Brown-paper-bag bug in 08c0d6ad: I missed one place that needed
to guard against RAINBOW arc colors.  Remarkably, nothing noticed
the invalid array access except buildfarm member thorntail.

Thanks to Noah Misch for assistance with tracking this down.

contrib/pg_trgm/trgm_regexp.c

@@ -1220,7 +1220,7 @@ addArcs(TrgmNFA *trgmNFA, TrgmState *state)
 		for (i = 0; i < arcsCount; i++)
 		{
 			regex_arc_t *arc = &arcs[i];
-			TrgmColorInfo *colorInfo = &trgmNFA->colorInfo[arc->co];
+			TrgmColorInfo *colorInfo;
 
 			/*
 			 * Ignore non-expandable colors; addKey already handled the case.
@@ -1228,8 +1228,14 @@ addArcs(TrgmNFA *trgmNFA, TrgmState *state)
 			 * We need no special check for WHITE or begin/end pseudocolors
 			 * here.  We don't need to do any processing for them, and they
 			 * will be marked non-expandable since the regex engine will have
-			 * reported them that way.
+			 * reported them that way.  We do have to watch out for RAINBOW,
+			 * which has a negative color number.
 			 */
+			if (arc->co < 0)
+				continue;
+			Assert(arc->co < trgmNFA->ncolors);
+
+			colorInfo = &trgmNFA->colorInfo[arc->co];
 			if (!colorInfo->expandable)
 				continue;