Fix: appending unallocated data from beyond exclusion entries

Issue
=====
If an exclusion string is smaller than the `LTTNG_SYMBOL_NAME_LEN`
integer, the `lttng_dynamic_buffer_append()` call will append
unallocated data to the buffer.

Fix
===
Use the `exclusion_len` value to copy the actual exclusion and pad the
remaining bytes with zeros.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I04c6681c28e82de29791541eb490158db9e503d0