gdb_server: check target examined while combining reg list

gdb_server: check target examined while combining reg list

Commit 6541233aa78d ("Combine register lists of smp targets.")
assumes that all the targets in the SMP cluster are already
examined and unconditionally call target_get_gdb_reg_list_noread()
that will in turn return error if the target is not examined yet.

Skip targets not examined yet.
Add an additional check in case the register list cannot be built,
e.g. because no target in the SMP cluster is examined. This should
never happen, but it's better to play safe.

Change-Id: I8609815c3d5144790fb05a870cb0c931540aef8a
Fixes: 6541233aa78d ("Combine register lists of smp targets.")
Reported-by: Michele Bisogno <michele.bisogno.ct@renesas.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6853
Tested-by: jenkins
Reviewed-by: Michele Bisogno <michele.bisogno.ct@renesas.com>
Reviewed-by: Tim Newsome <tim@sifive.com>