handle null mScanResult for CTS test

This adds one more assert statement to ensure the ScanResult
variable is not null before we proceed.

Bug: 27744749
TEST=run cts test without error

Change-Id: I48f3c5369792d3a55b594b71250b0dfdf0d5282b

tests/cts/net/src/android/net/wifi/cts/WifiManagerTest.java

@@ -296,6 +296,8 @@ public class WifiManagerTest extends AndroidTestCase {
         for (int i = 0; i < WIFI_SCAN_TEST_ITERATIONS; ++i) {
             startScan();
             // Make sure at least one AP is found.
+            assertTrue("mScanResult should not be null. This may be due to a scan timeout",
+                       mScanResults != null);
             assertFalse("empty scan results!", mScanResults.isEmpty());
             long nowMillis = SystemClock.elapsedRealtime();
             // Keep track of how many APs are fresh in one scan.