base/regdump.c: Be helpful about VFP/NEON registers before `regdump_init'.

On ARM32 (only), you really /must/ call `regdump_init' before dumping
VFP/NEON registers because otherwise there's no way to tell that they
need saving -- so they aren't and an important pointer is left null.

Rather than crashing, detect this and print a message explaining why the
register can't be dumped.