picture_image_device::get_bitmap can return a null reference

[ksherlock]

null references are not valid in C++ so compilers like to eliminate the null pointer check. This can result in a segfault,

eg, mame apple2e -gameio compeyes will segfault if no image (-pic) is provided.

switched to use a pointer instead.

[cuavas]

Since the MAME bitmap classes lazily allocate storage anyway, wouldn’t it be better to make picture_image_device always return a non-null bitmap and have the things that use it check .valid() on it instead? It would get rid of some indirection.

[ksherlock]

The m_bitmap member variable is never used outside those functions (a2bus_computereyes2_device::write_c0nx, apple2_compeyes_device::an0_w) so it could be eliminated and replaced with a local reference instead of converting to a pointer.