dhcp: Add buffer length and invalid cookie tests for DHCP options

Create an initial simple test program for these two cases.

src/libsystemd-dhcp/test-dhcp-option.c

@@ -0,0 +1,51 @@
+
+#include <stdio.h>
+#include <stdbool.h>
+#include <errno.h>
+#include <string.h>
+#include <assert.h>
+
+#include <util.h>
+
+#include "dhcp-protocol.h"
+#include "dhcp-internal.h"
+
+static void test_invalid_buffer_length(void)
+{
+        DHCPMessage message;
+
+        assert(dhcp_option_parse(&message, 0, NULL, NULL) == -EINVAL);
+        assert(dhcp_option_parse(&message, sizeof(DHCPMessage), NULL, NULL)
+               == -EINVAL);
+}
+
+static void test_cookie(void)
+{
+        DHCPMessage *message;
+        size_t len = sizeof(DHCPMessage) + 4;
+        uint8_t *opt;
+
+        message = malloc0(len);
+
+        opt = (uint8_t *)(message + 1);
+        opt[0] = 0xff;
+
+        assert(dhcp_option_parse(message, len, NULL, NULL) == -EINVAL);
+
+        opt[0] = 99;
+        opt[1] = 130;
+        opt[2] = 83;
+        opt[3] = 99;
+
+        assert(dhcp_option_parse(message, len, NULL, NULL) == -ENOMSG);
+
+        free(message);
+}
+
+int main(int argc, char *argv[])
+{
+        test_invalid_buffer_length();
+        test_cookie();
+
+        return 0;
+}