Fix #111811: Corrective smooth modifier crash with zero vertices

In the charge splash screen demo file for Blender 3.4, the corrective
smooth modifier is called with a zero vertex mesh, which means it ends
up trying to free a null pointer is creates locally. Checking for null
resolves the crash. A future fix will remove the assumption that
`rest_coords` are allocated with the guarded allocator and can be
used as an argument to `MEM_dupallocN`.

source/blender/modifiers/intern/MOD_correctivesmooth.cc

@@ -554,8 +554,8 @@ static void calc_deltas(CorrectiveSmoothModifierData *csmd,
     mul_v3_m3v3(csmd->delta_cache.deltas[l_index], imat, delta);
   }
 
-  MEM_freeN(tangent_spaces);
-  MEM_freeN(smooth_vertex_coords);
+  MEM_SAFE_FREE(tangent_spaces);
+  MEM_SAFE_FREE(smooth_vertex_coords);
 }
 
 static void correctivesmooth_modifier_do(ModifierData *md,