Template: Fix multi template graphs

The Bug:
It was not possible to create graphs with multi3 template graphs if different metrics or different numbers of metrics were given per graph.

Example (with 3 graphs):

graph 1: value, min, max, crit, warning
graph 2: value, crit, max, warning
graph 3: value, min

library/Graphite/Graphing/Template.php

@@ -207,8 +207,13 @@ protected function combineMetrics(
 
             $allowedNextCurveMetricsPerCurrentCurveName = [];
             foreach ($currentCombination as $currentCurveName => $currentCurveMetric) {
-                $allowedNextCurveMetricsPerCurrentCurveName[$currentCurveName]
-                    = $possibleCombinations[$currentCurveName][$nextCurveName][$currentCurveMetric];
+                if (isset($possibleCombinations[$currentCurveName][$nextCurveName][$currentCurveMetric])) {
+                    $allowedNextCurveMetricsPerCurrentCurveName[$currentCurveName]
+                        = $possibleCombinations[$currentCurveName][$nextCurveName][$currentCurveMetric];
+                } else {
+                    $metricsCombinations[] = $currentCombination;
+                    return;
+                }
             }
 
             $allowedNextCurveMetrics = $allowedNextCurveMetricsPerCurrentCurveName[$currentCurveName];