audio: Fix streamCommon nullptr check in VTS

Fix test code in AudioStream#GetStreamCommon test.
Avoid crashing if the HAL module returns a null
IStreamCommon instance pointer.

Bug: 205884982
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I118ea1df49a127780a901479cf8cd5787be601f7
diff --git a/audio/aidl/vts/VtsHalAudioCoreModuleTargetTest.cpp b/audio/aidl/vts/VtsHalAudioCoreModuleTargetTest.cpp
index 1a97004..d21b118 100644
--- a/audio/aidl/vts/VtsHalAudioCoreModuleTargetTest.cpp
+++ b/audio/aidl/vts/VtsHalAudioCoreModuleTargetTest.cpp
@@ -1835,10 +1835,10 @@
         ASSERT_NO_FATAL_FAILURE(stream.SetUp(module.get(), kDefaultBufferSizeFrames));
         std::shared_ptr<IStreamCommon> streamCommon1;
         EXPECT_IS_OK(stream.get()->getStreamCommon(&streamCommon1));
-        EXPECT_NE(nullptr, streamCommon1);
         std::shared_ptr<IStreamCommon> streamCommon2;
         EXPECT_IS_OK(stream.get()->getStreamCommon(&streamCommon2));
-        EXPECT_NE(nullptr, streamCommon2);
+        ASSERT_NE(nullptr, streamCommon1);
+        ASSERT_NE(nullptr, streamCommon2);
         EXPECT_EQ(streamCommon1->asBinder(), streamCommon2->asBinder())
                 << "getStreamCommon must return the same interface instance across invocations";
     }