Particles|Fixed: Crash due to null pointer access

Some of the particles created by the "lightning" hit effect didn't have
a current sector set, presumably because they were being spawned
outside the map.

Bug report: http://sourceforge.net/tracker/?func=detail&aid=3496963&group_id=74815&atid=542099#

doomsday/engine/portable/src/p_particle.c

@@ -1171,7 +1171,7 @@ static void P_MoveParticle(ptcgen_t* gen, particle_t* pt)
 
     // Check the new Z position only if not stuck to a plane.
     z = pt->pos[VZ] + pt->mov[VZ];
-    if(pt->pos[VZ] != DDMININT && pt->pos[VZ] != DDMAXINT)
+    if(pt->pos[VZ] != DDMININT && pt->pos[VZ] != DDMAXINT && pt->sector)
     {
         if(z > FLT2FIX(pt->sector->SP_ceilheight) - hardRadius)
         {

doomsday/engine/portable/src/rend_particle.c

@@ -300,7 +300,7 @@ static boolean populateSortBuffer(ptcgen_t* gen, void* context)
         float dist;
         porder_t* slot;
 
-        if(pt->stage < 0)
+        if(pt->stage < 0 || !pt->sector)
             continue;
 
         // Is the particle's sector visible?