Avoid crashing when failing graphics VTS

When the tests fail, ensure we continue running so we can run the
remainder of the tests.

Bug: 304976052
Test: atest GraphicsComposerAidlCommandTest
Change-Id: Ia6ab2bbd284bb7eddeb49164dcbe7285829ad9c3
diff --git a/graphics/composer/aidl/vts/VtsHalGraphicsComposer3_ReadbackTest.cpp b/graphics/composer/aidl/vts/VtsHalGraphicsComposer3_ReadbackTest.cpp
index 164e6d5..3f82925 100644
--- a/graphics/composer/aidl/vts/VtsHalGraphicsComposer3_ReadbackTest.cpp
+++ b/graphics/composer/aidl/vts/VtsHalGraphicsComposer3_ReadbackTest.cpp
@@ -85,6 +85,7 @@
     }
 
     void TearDown() override {
+        ASSERT_FALSE(mDisplays.empty());
         ASSERT_TRUE(mComposerClient->setPowerMode(getPrimaryDisplayId(), PowerMode::OFF).isOk());
         ASSERT_TRUE(mComposerClient->tearDown(mWriter.get()));
         mComposerClient.reset();