lib/bitmap: fix unchecked error while reading bytes
lib/bitmap: fix unchecked error while reading bytes

The `read_byte` method from `Reader` may return a negative int if an
error is encountered while reading.

This was not checked in lib/bitmap, we therefore add it.

Signed-off-by: Lucas Bajolet <7ee927989f658093d1603ebdd77063d48a82a2fc@gmail.com>

privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit