Even if ref_size is given on vrefbuffer's constructor, the minimum size of vrefbuffer::ref_buffer is 10.
Because int64, uint64, and double's msgpack expression size equals 9, and those buffer is allocated on the stack internally.
Projects that use -Wconversion don't compile due to errors in msgpack header
files. This commit fixes all the errors in msgpack-c for when -Wconversion is
enabled.
Now I can even compile msgpack-c itself with -Wconversion and all my projects:
CFLAGS=-Wconversion ./configure
make