lib: parameter check in hash_calculate

If hash_calculate is invoked with region_count = 0, it will try to hash
INT_MAX regions. We should check this parameter.

* Avoid a comparison with different signedness.
* Check that region_count is at least 1.
* Avoid a superfluous assignment.

Fixes: b37b46f0 ("rsa: Use checksum algorithms from struct hash_algo")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>