Fix: space left in buffer may be uninitilized on capacity increase

Fix: space left in buffer may be uninitilized on capacity increase

In the following case of dynamic buffer resize:

|---------|---------------------|------------------------|
          ^                     ^                        ^
 (a) original_size     (b) original_capacity     (c) new_capacity

The code (correctly) assumes that the space between b and c is
zero-initialized. However, the space between a and b will be left
uninitialized.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>