render_con: initialize int sizes[children]

This fixes a clang-analyzer warning

src/render.c

@@ -202,6 +202,7 @@ void render_con(Con *con, bool render_fullscreen) {
 
     /* precalculate the sizes to be able to correct rounding errors */
     int sizes[children];
+    memset(sizes, 0, children*sizeof(int));
     if ((con->layout == L_SPLITH || con->layout == L_SPLITV) && children > 0) {
         assert(!TAILQ_EMPTY(&con->nodes_head));
         Con *child;