util/nvmutil: add guard in rhex()

i removed this before, but it's good to put it
here defensively, in case i ever mess up
the urandom read function again.

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

util/nvmutil/nvmutil.c

@@ -973,6 +973,9 @@ rhex(void)
 #else
 		n = (size_t)read_dev_urandom(
 		    urandom_fd, rnum, sizeof(rnum));
+
+		if (!n || n > sizeof(rnum))
+			err(ECANCELED, "Randomisation failure");
 #endif
 	}