gdbserver: Add assert in find_register_by_number

It helped me during development, catching bugs closer to when they actually
happened.

Also remove the equivalent gdb_assert in regcache_raw_read_unsigned, since
it's checking the same condition a few frames above.

Suggested-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>