The first problem is the while loop around the code that processes prestate.
That's wrong because there may be a need to read more data before continuing
to process a prestate.
The second problem is the code assumption that a prestate will be processed
at once, and then unconditionally process the current state.
Both problems are likely to happen when reading a large buffer because more
than one read may be required.
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>