[SPARK-13416][GraphX] Add positive check for option 'numIter' in StronglyConnectedComponents

JIRA: https://issues.apache.org/jira/browse/SPARK-13416

## What changes were proposed in this pull request?

The output of StronglyConnectedComponents with numIter no greater than 1 may make no sense. So I just add require check in it.

## How was the this patch tested?

 unit tests passed

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #11284 from zhengruifeng/scccheck.

graphx/src/main/scala/org/apache/spark/graphx/lib/StronglyConnectedComponents.scala

@@ -36,7 +36,7 @@ object StronglyConnectedComponents {
    * @return a graph with vertex attributes containing the smallest vertex id in each SCC
    */
   def run[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED], numIter: Int): Graph[VertexId, ED] = {
-
+    require(numIter > 0, s"Number of iterations ${numIter} must be greater than 0.")
     // the graph we update with final SCC ids, and the graph we return at the end
     var sccGraph = graph.mapVertices { case (vid, _) => vid }
     // graph we are going to work with in our iterations