libsystemd-network: fix dhcp option buffer confusion

We were writing to the wrong buffer with a wrong offset :(
Bug present since the original introduction of the code in
04b28be1a306fd2ba454d3ee333d63df71aa3873.

meson.build

@@ -802,7 +802,7 @@ if not meson.is_cross_build()
                 id = id_result.stdout().to_int()
                 if id != 65534
                         warning('\n' +
-                                'The local group with the configured group name "@0@" of the nobody group does not have UID 65534 (it has @1@).\n'.format(nobody_group, id) +
+                                'The local group with the configured group name "@0@" of the nobody group does not have GID 65534 (it has @1@).\n'.format(nobody_group, id) +
                                 'Your build will result in an group table setup that is incompatible with the local system.')
                 endif
         endif

src/libsystemd-network/dhcp-option.c

@@ -17,6 +17,7 @@
 static int option_append(uint8_t options[], size_t size, size_t *offset,
                          uint8_t code, size_t optlen, const void *optval) {
         assert(options);
+        assert(size > 0);
         assert(offset);
 
         if (code != SD_DHCP_OPTION_END)
@@ -165,7 +166,7 @@ int dhcp_option_append(DHCPMessage *message, size_t size, size_t *offset,
                         } else if (r == -ENOBUFS && use_sname) {
                                 /* did not fit, but we have more buffers to try
                                    close the file array and move the offset to its end */
-                                r = option_append(message->options, size, offset, SD_DHCP_OPTION_END, 0, NULL);
+                                r = option_append(message->file, sizeof(message->file), &file_offset, SD_DHCP_OPTION_END, 0, NULL);
                                 if (r < 0)
                                         return r;