databases/rrdtool: fix segmentation fault

Fix a segmentation fault in rrd_graph:print_calc when vidx is -1 on GF_VRULE
Pull in upstream patch to fix the issue.
This fixes upstream bug #1078

PR:		244808
Submitted by:	John W. O'Brien

databases/rrdtool/Makefile

@@ -3,7 +3,7 @@
 
 PORTNAME=	rrdtool
 PORTVERSION=	1.7.2
-PORTREVISION=	1
+PORTREVISION=	2
 CATEGORIES=	databases graphics
 MASTER_SITES=	http://oss.oetiker.ch/rrdtool/pub/
 

databases/rrdtool/files/patch-src_rrd__graph__helper.c

@@ -0,0 +1,25 @@
+Backport of:
+
+commit 254e4e44cd7828fba8f5bb18a7a29668221e0af5
+Author: John W. O'Brien <john@saltant.com>
+Date:   Sat Mar 14 08:49:47 2020 -0400
+
+    Fix double meaning of time 0 as uninitialized value
+
+    Treat "VRULE:0#..." as "VRULE:1#..." because elsewhere xrule==0 is
+    used to mean xrule has not been set, which can lead to an array
+    bounds violation.
+
+--- src/rrd_graph_helper.c.orig	2020-03-14 17:06:47 UTC
++++ src/rrd_graph_helper.c
+@@ -776,6 +776,10 @@ static graph_desc_t* newGraphDescription(image_desc_t 
+               }
+               if (gf==GF_VRULE){
+                  gdp->xrule=val;
++                 if (gdp->xrule == 0) {
++                    /* distinguish from uninitialized */
++                    gdp->xrule++;
++                 }
+               }
+               else {
+                  gdp->yrule=val;