coccinelle: add reallocarray() coccinelle script

Let's systematically make use of reallocarray() whereever we invoke
realloc() with a product of two values.

src/basic/prioq.c

@@ -173,7 +173,7 @@ int prioq_put(Prioq *q, void *data, unsigned *idx) {
                 struct prioq_item *j;
 
                 n = MAX((q->n_items+1) * 2, 16u);
-                j = realloc(q->items, sizeof(struct prioq_item) * n);
+                j = reallocarray(q->items, n, sizeof(struct prioq_item));
                 if (!j)
                         return -ENOMEM;