Fix use of pointer in write(to::IO, from::GenericIOBuffer)

[nhz2]

Fixes #54636

With the fixes in #57570, as far as I can tell, this is the last place where GenericIOBuffer assumes data is stored contiguously in memory.

[jakobnissen]

Good catch. Also good with the from.readable check. 👍

[jakobnissen]

Just to be clear: GenericIOBuffer is private, and as far as I can tell, all the public constructors create buffers backed by Memory{UInt8}. So, this is not really a bugfix, but may make the code more robust.
There is a case to be made that GenericIOBuffer could be made un-generic, making it easier to reason about, and test. The genericness does seem like genericness for the sake of genericness - there is, AFAICT, no actual use case for having anything other than Memory{UInt8}.

Edit: That was completely wrong, not sure how I missed this:

julia> typeof(IOBuffer(0x01:0x02))
Base.GenericIOBuffer{UnitRange{UInt8}}