fix(qudarap): guard null scint dereference on GPU reemission path (#294)

PR #291 correctly made missing scintillation ICDF non-fatal for valid
non-scintillating geometries. This PR addresses a separate case: a
problematic material configuration where `REEMISSIONPROB > 0` is
present, but no emission spectrum is defined.

In the current implementation, the boundary texture still carries a
nonzero reemission probability from `qudarap/qbnd.h:195`, even when no
`QScint`/ICDF is created because `FASTCOMPONENT` and `SLOWCOMPONENT` are
missing. During propagation, a photon can then enter the reemission
branch in `qudarap/qsim.h:804` and attempt to sample a reemission
wavelength via `scint->wavelength()` in `qudarap/qsim.h:812`, which
becomes a null dereference on the GPU.

This PR guards that path by treating reemission-without-`QScint` as bulk
absorption instead of attempting to reemit. It also guards the
scintillation genstep path against a null `scint` pointer.

A reproducer geometry is included in
`tests/geom/opticks_raindrop_reemit_no_scint.gdml`.

In short:

- No scintillation properties at all: valid non-scintillating geometry,
handled by PR #291
- `REEMISSIONPROB > 0` but no emission spectrum: inconsistent
configuration, now guarded to avoid GPU null dereference

---------

Co-authored-by: ggalgoczi <ggalgoczi@users.noreply.github.com>
Co-authored-by: Dmitri Smirnov <dmixsmi@gmail.com>

Author: ggalgoczi

qudarap/qsim.h

@@ -803,6 +803,12 @@ inline QSIM_METHOD int qsim::propagate_to_boundary(unsigned& flag, RNG& rng, sct
 
             if (u_reemit < reemission_prob)
             {
+                if (scint == nullptr)
+                {
+                    flag = BULK_ABSORB;
+                    return BREAK;
+                }
+
                 float u_re_wavelength = curand_uniform(&rng);
                 float u_re_mom_ph = curand_uniform(&rng);
                 float u_re_mom_ct = curand_uniform(&rng);
@@ -2546,7 +2552,9 @@ inline QSIM_METHOD void qsim::generate_photon(sphoton& p, RNG& rng, const quad6&
 
         case OpticksGenstep_DsG4Scintillation_r4695:
         case OpticksGenstep_SCINTILLATION:
-                                              scint->generate(        p, rng, gs, photon_id, genstep_id ) ; break ;
+            if (scint != nullptr)
+                scint->generate(p, rng, gs, photon_id, genstep_id);
+            break;
 
         case OpticksGenstep_INPUT_PHOTON:    { p = evt->photon[photon_id] ; p.set_flag(TORCH) ; }        ; break ;
         default:                             generate_photon_dummy(  p, rng, gs, photon_id, genstep_id)  ; break ;