`Cstring` does not check for null-termination

[Ismael-VC]

This is pasted verbatim from @ScottPJones report at julia-dev:

• https://groups.google.com/forum/#!topic/julia-dev/gYrmxNkc5H0

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In the course of adding support to CSV.jl for being able to load into arbitrary AbstractFloat types, via a keyword argument floattype (which defaults to Float64),
I ran into a number of bugs (both in v0.4.x and v0.5-dev). The problem occurs because pointer_to_string() does not create a string that has a terminating \0 (which I feel is correct),
but Cstring does not ensure that a string passed to C has a terminating \0.
This is inconsistent with Cwstring, which does make sure that a Cwstring does have a terminating Cwchar(0).

I'd make a PR to fix these bugs, but since I'm unable to, I'm posting the issue here.

Here are some test cases demonstrating the bugs:

julia> s = "53.9abcdef"
"53.9abcdef"

julia> u = pointer_to_string(pointer(s.data),2)
"53"

julia> t = pointer_to_string(pointer(s.data),4)
"53.9"

julia> parse(Dec64, t)
ERROR: ArgumentError: invalid number format 53.9
 in parse at /j/DecFP.jl/src/DecFP.jl:82

julia> parse(Dec64, u)
ERROR: ArgumentError: invalid number format 53
 in parse at /j/DecFP.jl/src/DecFP.jl:82

julia> parse(BigInt, u)
ERROR: ArgumentError: invalid BigInt: "53"
 in tryparse_internal at gmp.jl:104
 in parse at parse.jl:146

julia> parse(BigFloat, t)
ERROR: ArgumentError: invalid number format "53.9" for BigFloat
 in parse at parse.jl:164