Simpler handling of errors when creating tbc variables

New field 'lua_State.ptbc' keeps to-be-closed variable until its
upvalue is created, so that it can be closed in case of a
memory-allocation error.

manual/manual.of

@@ -4358,10 +4358,6 @@ nor modified before a corresponding call to @Lid{lua_closeslot}.
 This function should not be called for an index
 that is equal to or below an active to-be-closed index.
 
-In the case of an out-of-memory error,
-the value in the given index is immediately closed,
-as if it was already marked.
-
 Note that, both in case of errors and of a regular return,
 by the time the @idx{__close} metamethod runs,
 the @N{C stack} was already unwound,