random: Small patch for consistency and really burn the stack.

* random/rndlinux.c (_gcry_rndlinux_gather_random): s/int/size_t/.
(_gcry_rndlinux_gather_random): Replace memset by wipememory.

size_t was suggested by Marcus Meissner <meissner@suse.de>. While
looking at the code I identified the useless (i.e. likely optimized
away) memset.