Clean-up: unchecked return value

The return value of lttng_dynamic_buffer_set_size() is ignored in
lttng_dynamic_array_clear(). It is okay as
lttng_dynamic_buffer_set_size(..., 0) can't fail. However, it
results in a Coverity Scan warning.

1412201 Unchecked return value

If the function returns an error value, the error value may be
mistaken for a normal value.

In lttng_dynamic_array_clear: Value returned from a function is not
checked for errors before being used (CWE-252)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I1943e299e4cb5752711f2d25b0dd9a5c44a28a18