video: fbdev: uvesafb: Fix string related warnings

Two W=1 string related warnings.
- Using strncpy to copy string without null-termination generates a
  warning.  Use memcpy to copy only the relevant chars

- Fix a potential bug with a very long string, subtract one from the
  length to make room for the termination null.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Michal Januszewski <spock@gentoo.org>
Cc: linux-fbdev@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-27-sam@ravnborg.org