[mpp_buffer]: return 0 size when input MppBuffer is NULL

git-svn-id: https://10.10.10.66:8443/svn/MediaProcessPlatform/trunk/mpp@1063 6e48237b-75ef-9749-8fc9-41990f28c85a

mpp/base/mpp_buffer.cpp

@@ -181,7 +181,7 @@ size_t mpp_buffer_get_size(MppBuffer buffer)
 {
     if (NULL == buffer) {
         mpp_err_f("invalid NULL input\n");
-        return -1;
+        return 0;
     }
 
     MppBufferImpl *p = (MppBufferImpl*)buffer;