recently added more reads into that function, if a later read fails, i must
either free what's already allocated, or return the pointer so that the free
callback will release it.
looks like each platform implements long double differently (different bit count)
so we can't save them as binary, and we also want to avoid creating a new RDB
format version, so we save these are hex strings using "%La".
This commit includes a change in the arguments of ld2string to support this.
as well as tests for coverage and short reads.
coded by @guybe7
