transport/server.cc: Return correct size of decompressed lz4 buffer

An incorrect size is returned from the function, which could lead to
crashes or undefined behavior. Fix by erroring out in these cases.

Fixes #11476

transport/server.cc

@@ -727,7 +727,10 @@ future<fragmented_temporary_buffer> cql_server::connection::read_and_decompress_
                     if (ret < 0) {
                         throw std::runtime_error("CQL frame LZ4 uncompression failure");
                     }
-                    return out.size();
+                    if (ret != out.size()) {
+                        throw std::runtime_error("Malformed CQL frame - provided uncompressed size different than real uncompressed size");
+                    }
+                    return static_cast<size_t>(ret);
                 });
                 on_compression_buffer_use();
                 return uncomp;