CCodec: Fix null check

'dst' is always non-null, by assumption of the function.  We need
to check if "*dst" is null or not.

We also update the interface definition.

Bug: 197719179
Test: TreeHugger
Change-Id: Ic86891f936f4e84241c55674c1aedcb0d7e15483
diff --git a/media/codec2/sfplugin/CCodecBuffers.cpp b/media/codec2/sfplugin/CCodecBuffers.cpp
index e12b209..d57eb0c 100644
--- a/media/codec2/sfplugin/CCodecBuffers.cpp
+++ b/media/codec2/sfplugin/CCodecBuffers.cpp
@@ -233,7 +233,7 @@
     if (!srcBuffer) {
         return false;
     }
-    if (!dst) {
+    if (!*dst) {
         *dst = new Codec2Buffer(
                 mFormat,
                 new ABuffer(mDataConverter->targetSize(srcBuffer->size())));