util/nvmutil: rhex(): fail if errno not zero

The code was only checking whether all of the bytes were read,
but there are other errors that can be caught via errno.

Enforce strict errno handling, when generating random
numbers for command `setmac`.

util/nvmutil/nvmutil.c

@@ -258,6 +258,8 @@ rhex(void)
 			warn("%s", "/dev/urandom");
 			return 16;
 		}
+		if ((errno != 0))
+			return 16;
 	}
 
 	return rbuf[rindex++] & 0xf;