Fix: bytecode interpreter context_get_index() leaves byte order uninitialized

Fix: bytecode interpreter context_get_index() leaves byte order uninitialized

Observed Issue
==============

With lttng-ust 2.13, when using the event notification capture feature
to capture a context field, e.g. '$ctx.cpu_id', the captured value is
often observed in reverse byte order.

This issue is not visible in lttng-ust 2.12 because it does not
implement the event notification capture feature. However, it would
become observable if a lttng-tools emits a filter bytecode
BYTECODE_OP_GET_SYMBOL instruction to load the context value. For
compatibility purposes, lttng-tools only uses
BYTECODE_OP_GET_CONTEXT_REF to load the filter context fields,
but nothing prevents a future lttng-tools version from using
BYTECODE_OP_GET_SYMBOL instead.

Cause
=====

Within the bytecode interpreter, context_get_index() leaves the "rev_bo"
field uninitialized in the top of stack.

Solution
========

Initialize the rev_bo field based on the context field type
reserve_byte_order field.

Known drawbacks
===============

None.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I74996d501cee3c269658d98dfc0d0050b74c5ddb