codec2: BufferPoolClient: fix NPE in fetchBufferHandle if connection is null

Do a null pointer check before connection->fetch().

Bug: 344846184
Flag: EXEMPT bugfix
Test:
1.atest android.media.decoder.cts.AdaptivePlaybackTest
2.atest android.media.decoder.cts.DecoderTest

Change-Id: I30031b4abf46f920b708320ffa3fa9d1598027b4
diff --git a/media/module/bufferpool/2.0/BufferPoolClient.cpp b/media/module/bufferpool/2.0/BufferPoolClient.cpp
index cda23ff..66d11fa 100644
--- a/media/module/bufferpool/2.0/BufferPoolClient.cpp
+++ b/media/module/bufferpool/2.0/BufferPoolClient.cpp
@@ -762,6 +762,10 @@
     } else {
         connection = mRemoteConnection;
     }
+    if (!connection) {
+        ALOGE("connection null: fetchBufferHandle()");
+        return ResultStatus::CRITICAL_ERROR;
+    }
     ResultStatus status;
     Return<void> transResult = connection->fetch(
             transactionId, bufferId,