Fix T95212: Mirror modifier normals crash

The vertex and face normals from the input mesh
were used to calculate the normals on the result,
which could cause a crash because the result should
be about twice as large.

Also remove an unnecessary dirty tag, since it is handled
automatically when creating a new mesh or in the case
of the mirror modifier, when calculating the new custom
face corner normals.

source/blender/modifiers/intern/MOD_mirror.c

@@ -120,9 +120,6 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
 
   result = mirrorModifier__doMirror(mmd, ctx->object, mesh);
 
-  if (result != mesh) {
-    BKE_mesh_normals_tag_dirty(result);
-  }
   return result;
 }