Clean-up: simplify the implementation of dynamic buffer set_capacity

Only use realloc() to implement set_capacity's logic. In the case
where buf is NULL, realloc acts like malloc() anyhow.

Moreover, the memory does not need to be zeroed on allocation since
size increases provide this guarantee.

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