fix maybe-uninitialized error in serializer.cpp

fix maybe-uninitialized error in serializer.cpp

Function Serializer::serialize(const std::string config_path, size_t &size) can
return nullptr in case of error, thus leaving size to be uninitialized.

[   50s] src/internal/serializer.cpp: In member function 'serialize':
[   50s] src/internal/serializer.cpp:133:50: error: 'size' may be used uninitialized in this function [-Werror=maybe-uninitialized]
[   50s]   output.write(reinterpret_cast<char *>(buf), size);
[   50s]                                                   ^
[   50s] src/internal/serializer.cpp:130:9: note: 'size' was declared here
[   50s]   size_t size

Change-Id: If8b2c735e6b01e73b592827b937e0dfe8f94a615
Signed-off-by: Himanshu Maithani <himanshu.m@samsung.com>