util/nvmutil: reset errno on /dev/random fallback

i didn't catch this before. stale error state would
have passed through, even on ultimate success.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -358,7 +358,20 @@ open_dev_urandom(void)
 		/*
 		 * Fall back to /dev/random on old platforms
 		 * where /dev/urandom does not exist.
+		 *
+		 * In general, we can assume that any system
+		 * that would have urandom, probably has the
+		 * other device (/dev/urandom).
+		 *
+		 * Given that on many modern systems, urandom
+		 * and random are essentially the same, this
+		 * also provides some fault-tolerance there,
+		 * while still ultimately being strict under
+		 * fallback conditions, thus:
 		 */
+
+		errno = 0;
+
 		rname = oldrandom;
 		xopen(&rfd, rname, O_RDONLY, &st_rfd);
 	}