Fix bug in display of peak memory usage under Manage Crawl, a=chris

diff --git a/src/models/CrawlModel.php b/src/models/CrawlModel.php
index 0319606dd..ce416f136 100755
--- a/src/models/CrawlModel.php
+++ b/src/models/CrawlModel.php
@@ -1540,9 +1540,10 @@ EOT;
                 $memory_fields = ["WEBAPP_PEAK_MEMORY", "FETCHER_PEAK_MEMORY",
                     "QUEUE_PEAK_MEMORY", "SCHEDULER_PEAK_MEMORY"];
                 foreach ($memory_fields as $field) {
+                    $a_status_data[$field] = (isset($a_status_data[$field])) ?
+                        $a_status_data[$field] : 0;
                     $status[$channel][$field] =
-                        (!isset($a_status[$field])) ? 0 :
-                        max($status[$channel][$field], $a_status[$field]);
+                        max($status[$channel][$field], $a_status_data[$field]);
                 }
             }
         }