Fix: bytecode validator: reject specialized load field/context ref instructions

Reject specialized load ref and get context ref instructions so a
bytecode crafted with nefarious intent cannot read a memory area larger
than the memory targeted by the instrumentation.

This prevents bytecode received from the session daemon from performing
out of bound memory accesses and from disclosing the content of
application memory beyond what has been targeted by the instrumentation.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ica16b97167d391d86e73b05fbf0210ff52b9c9f1