util/nvmutil: rhex: don't read twice!

we currently never read the 0th byte, so if we need
all 12, and we do when every byte is random, we
read again just to get one byte.

not really a bug, but it is a performance penalty,
so let's fix it!

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

util/nvmutil/nvmutil.c

@@ -395,10 +395,10 @@ hextonum(char ch)
 static uint8_t
 rhex(void)
 {
-	static uint8_t n = 0;
+	static int n = -1;
 	static uint8_t rnum[12];
 
-	if (!n) {
+	if (n == -1) {
 		n = sizeof(rnum) - 1;
 		read_file_PERFECTLY_or_die(rfd, rnum, sizeof(rnum),
 		    0, "/dev/urandom", NULL);