decorations: use calloc() instead of malloc()

In cases where it makes sense, use calloc() instead of malloc():

* When allocating large objects with many members, many for which
  NULL/0 is a good default value.
* Arrays etc where we explicitly initialize to NULL anyway.

decorations/underline.c

@@ -31,11 +31,11 @@ expose(const struct deco *deco, cairo_t *cr, int x, int y, int width, int height
 static struct deco *
 underline_new(int size, struct rgba color)
 {
-    struct private *priv = malloc(sizeof(*priv));
+    struct private *priv = calloc(1, sizeof(*priv));
     priv->size = size;
     priv->color = color;
 
-    struct deco *deco = malloc(sizeof(*deco));
+    struct deco *deco = calloc(1, sizeof(*deco));
     deco->private = priv;
     deco->expose = &expose;
     deco->destroy = &destroy;