libdwfl: Handle unwind frame when the return address register isn't set.

libdwfl: Handle unwind frame when the return address register isn't set.

When we have unwound the frame and then cannot set the return address
we wouldn't set any error. That meant that a dwfl_thread_getframes ()
call could end in an error, but without any dwfl_errno set, producing
the "no error" error message.

If we cannot set the return address at the end of unwinding the frame
that means that either the return address register is bogus (error),
or that the return address is undefined (end of the call stack).

This fixes the run-backtrace-native-biarch.sh testcase for me on an
i386 on x86_64 setup with gcc 7.2.1 and glibc 2.17.

Signed-off-by: Mark Wielaard <mark@klomp.org>