datarate test: add explicit braces.

BUG=aomedia:2386

Change-Id: Idd965c51e22b46628f02fd508780d57de8bff672
diff --git a/test/datarate_test.cc b/test/datarate_test.cc
index 9cc6f78..b1f8fe8 100644
--- a/test/datarate_test.cc
+++ b/test/datarate_test.cc
@@ -215,11 +215,12 @@
           << " The datarate for the file is lower than target by too much!";
       ASSERT_LE(effective_datarate_, cfg_.rc_target_bitrate * 1.15)
           << " The datarate for the file is greater than target by too much!";
-      if (last_drop > 0)
+      if (last_drop > 0) {
         ASSERT_LE(first_drop_, last_drop)
             << " The first dropped frame for drop_thresh " << i
             << " > first dropped frame for drop_thresh "
             << i - kDropFrameThreshTestStep;
+      }
       ASSERT_GE(num_drops_, last_num_drops * 0.85)
           << " The number of dropped frames for drop_thresh " << i
           << " < number of dropped frames for drop_thresh "