Only reduce the number of cores once when removing an executor

core/src/main/scala/spark/deploy/master/ApplicationInfo.scala

@@ -52,9 +52,11 @@ private[spark] class ApplicationInfo(
   }
 
   def removeExecutor(exec: ExecutorInfo) {
+    if (executors.contains(exec.id)) {
       executors -= exec.id
       coresGranted -= exec.cores
     }
+  }
 
   def coresLeft: Int = desc.maxCores - coresGranted