exercises: examples: improve ArrayList deinits

[ee7]

There should be an errdefer after these initializations of an ArrayList - we subsequently use toOwnedSlice.

From the doc comment of toOwnedSlice:

/// The caller owns the returned memory. Empties this ArrayList,
/// Its capacity is cleared, making deinit() safe but unnecessary to call.
pub fn toOwnedSlice(self: *Self) Allocator.Error!Slice {

Refs: #313