Fix T47367: Segfault in BIF_draw_manipulator() with "around active" and the active object hidden

Accidentally left off null check when cleaning up the code there in
34993bf97dcbfd29289d14228ac10f1cfa206a15  (GPencil_Editing_Stage3 branch)

Thanks to Sami Pelkonen (pltsi) who reported this bug, along with
some nice and detailed backtraces which made it easy to locate
the problem :)

source/blender/editors/transform/transform_manipulator.c

@@ -1633,7 +1633,7 @@ void BIF_draw_manipulator(const bContext *C)
 
 				if (((v3d->around == V3D_AROUND_ACTIVE) && (scene->obedit == NULL)) &&
 				    ((gpd == NULL) || !(gpd->flag & GP_DATA_STROKE_EDITMODE)) &&
-				    (!(ob->mode & OB_MODE_POSE)))
+				    (ob && !(ob->mode & OB_MODE_POSE)))
 				{
 					copy_v3_v3(rv3d->twmat[3], ob->obmat[3]);
 				}