MSBuild warnings in serdata.cpp

[brawner]

Bug report

Required Info:
Building on Windows 10, 1903 in a docker container with the latest VCTools (16.24) installed from VS BuildTools.
https://citest.ros2.org/job/nightly_win_extra_rmw_rel/10/warnings43Result/

I get a couple of MSBuild warnings,
serdata.cpp:115

'initializing': conversion from 'size_t' to 'uint32_t', possible loss of data

Looks to be introduced in #77

serdata.cpp:506

unary minus operator applied to unsigned type, result still unsigned

Introduced in #75

@rotu

[brawner]

Looks like these warnings have been around for awhile though. https://ci.ros2.org/job/ci_windows/8800/