libdwfl: only use thread->unwound for initial frame

libdwfl: only use thread->unwound for initial frame

thread->unwound is only used for set_initial_registers (via
dwfl_thread_state_registers, dwfl_thread_state_register_pc, and a
special case in core_set_initial_registers). At that point,
thread->unwound is always the initial frame, so there's no need to
update it as we unwind the stack. Let's set it to NULL after we do the
initial setup. This simplifies the next change.

Signed-off-by: Omar Sandoval <osandov@fb.com>