ralloc: Use strnlen() inside of strncat()

ralloc: Use strnlen() inside of strncat()

If the str is long or isn't null-terminated, strlen() could take a lot
of time or even crash. I don't know why was it used in the first place,
maybe for platforms without strnlen(), but strnlen() is already used
inside of ralloc_strndup(), so this change should not additionally
break anything.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>