arch: riscv: stacktrace: fix output without `ra` on the stack top

Account for the scenario when we are doing `esf`-based
unwinding from a function which doesn't have any callee.
In this case the `ra` is not saved on the stack and the
second function from the top of the frame could be missing.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>