common: introduce bt_g_array_index

common: introduce bt_g_array_index

Introduce bt_g_array_index, a wrapper to g_array_index, that adds a
check that the specified index does not go past the end of the array.

Use BT_ASSERT_DBG, so that it's enabled only in builds with debug mode
enabled.

The difficulty is that g_array_index is expected to yield an lvalue,
since some call sites take the address of what it yields, or assign to
it.  The way I found to make it work is to add a statement expression
that has no effect, and put the BT_ASSERT_DBG in it.

Change-Id: I280c0657ec6f1cadd01328eec79da5cefada5c31
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10905
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>