Fix bz175: Pile mark is shown when potion is destroyed

Fix bz175: Pile mark is shown when potion is destroyed

Original bug report:

> When killing something that's carrying a potion, or death-drops a potion,
> or stands on top of a potion, with a force bolt or a wand of striking,
> "you hear something shatter" or "a potion of foo shatters" but the corpse
> is inverse as if it's (still) a pile.

Unfortunately the newsym() checks for already existing glyph, and
the gbuf doesn't distinguish between object piles and single items,
so newsym doesn't mark the location for update.

This is a dirty hack to force the newsym to update the glyph.
The glyph buffering should be revisited in a future version.