Fix: Prevent segfault in riscv_invalidate_register_cache for non-examined targets. (#692)

The segfault could be triggered if:

- At least one target failed to get examined (therefore does not have the
  register cache set up yet),

- and "reset" TCL command was issued, which internally tries to
  invalidate the register cache.

Minor cleanup: "registers_initialized" member removed from riscv_info_t
because it is not used anywhere.

Change-Id: I6288c0d4343ef6a330fb2a6b49d388e7eafa32a2
Signed-off-by: Jan Matyas <matyas@codasip.com>