util/nvmutil: extra part check in set_checksum

it calls word() anyway, but we should still check it here,
since this is quite a critical function.

the other bound checks are done by word(), which this
function uses to add everything up.

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

util/nvmutil/nvmutil.c

@@ -617,6 +617,9 @@ set_checksum(size_t p)
 	size_t c;
 	uint16_t val16 = 0;
 
+	if (p > 1)
+		err(ECANCELED, "Bad part num %zu (must be 0 or 1)", p);
+
 	for (c = 0; c < NVM_CHECKSUM_WORD; c++)
 		val16 += word(c, p);