dummyflasher.c: Replace atoi() with strtoul()

BUG=none
BRANCH=none
TEST=builds

Change-Id: Ib9e66016a2f4ce2d13b833c261f900cab80916b7
Spotted-by: Angel Pons <th3fanbus@gmail.com>
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/54854
Reviewed-by: Sam McNally <sammc@google.com>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/dummyflasher.c b/dummyflasher.c
index 2f39c98..4661f3d 100644
--- a/dummyflasher.c
+++ b/dummyflasher.c
@@ -668,6 +668,7 @@
 #if EMULATE_CHIP
 	struct stat image_stat;
 #endif
+	char *endptr;
 
 	struct emu_data *data = calloc(1, sizeof(struct emu_data));
 	if (!data) {
@@ -710,9 +711,9 @@
 
 	tmp = extract_programmer_param("spi_write_256_chunksize");
 	if (tmp) {
-		data->spi_write_256_chunksize = atoi(tmp);
+		data->spi_write_256_chunksize = strtoul(tmp, &endptr, 0);
 		free(tmp);
-		if (data->spi_write_256_chunksize < 1) {
+		if (*endptr != '\0' || data->spi_write_256_chunksize < 1) {
 			msg_perr("invalid spi_write_256_chunksize\n");
 			return 1;
 		}
@@ -977,7 +978,6 @@
 #ifdef EMULATE_SPI_CHIP
 	status = extract_programmer_param("spi_status");
 	if (status) {
-		char *endptr;
 		errno = 0;
 		data->emu_status = strtoul(status, &endptr, 0);
 		free(status);