util/nvmutil: fix bad bound check

the current check is too liberal. make it sticter.

the issue is that the previous check did not take
into account that it's a check on a uint16_t array,
against nf which refers to a number of bytes.

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

util/nvmutil/nvmutil.c

@@ -425,7 +425,7 @@ check_bounds(int c, int p)
 {
 	if ((p != 0) && (p != 1))
 		err(SET_ERR(EINVAL), "check_bounds: invalid partnum %d", p);
-	if ((c < 0) || (c > nf))
+	if ((c < 0) || (c > ((nf >> 1) - 1)))
 		err(SET_ERR(EINVAL), "check_bounds: out of bounds %d", c);
 }