misc::vec_with_size() can drop uninitialized memory if clone panics

misc::vec_with_size creates a vector of the provided size and immediately
calls vec.set_len(size) on it, initially filling it with uninitialized memory.
It then inserts elements using vec[i] = value.clone().

If the value.clone() call panics, uninitialized items in the vector will be
dropped leading to undefined behavior.