bootchart: add assert

since 376cd3b89c LIST_FIND_TAIL accepts
an empty list. That removed an assert in LIST_FIND_TAIL and we now
theoretically risk a null pointer deref. This adds the assert directly
to protect against that.

src/bootchart/svg.c

@@ -80,6 +80,8 @@ static void svg_header(void) {
         double h;
         struct list_sample_data *sampledata_last;
 
+        assert(head);
+
         sampledata = head;
         LIST_FIND_TAIL(link, sampledata, head);
         sampledata_last = head;