push_cloned can drop uninitialized memory or double free on panic

Affected versions of stack_dst used a push_inner function that increased
the internal length of the array and then called val.clone().

If the val.clone() call panics, the stack could drop an already dropped
element or drop uninitialized memory.

This issue was fixed in 2a4d538 by increasing the length of the array after
elements are cloned.