cocci: use ALLOC_ARRAY

Add a semantic patch for using ALLOC_ARRAY to allocate arrays and apply
the transformation on the current source tree.  The macro checks for
multiplication overflow and infers the element size automatically; the
result is shorter and safer code.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/array.cocci

@@ -24,3 +24,19 @@ expression n;
 @@
 - memcpy(dst, src, n * sizeof(T));
 + COPY_ARRAY(dst, src, n);
+
+@@
+type T;
+T *ptr;
+expression n;
+@@
+- ptr = xmalloc(n * sizeof(*ptr));
++ ALLOC_ARRAY(ptr, n);
+
+@@
+type T;
+T *ptr;
+expression n;
+@@
+- ptr = xmalloc(n * sizeof(T));
++ ALLOC_ARRAY(ptr, n);

worktree.c

@@ -175,7 +175,7 @@ struct worktree **get_worktrees(unsigned flags)
 	struct dirent *d;
 	int counter = 0, alloc = 2;
 
-	list = xmalloc(alloc * sizeof(struct worktree *));
+	ALLOC_ARRAY(list, alloc);
 
 	list[counter++] = get_main_worktree();