Merge branch 'ab/free-and-null'

A common pattern to free a piece of memory and assign NULL to the
pointer that used to point at it has been replaced with a new
FREE_AND_NULL() macro.

* ab/free-and-null:
  *.[ch] refactoring: make use of the FREE_AND_NULL() macro
  coccinelle: make use of the "expression" FREE_AND_NULL() rule
  coccinelle: add a rule to make "expression" code use FREE_AND_NULL()
  coccinelle: make use of the "type" FREE_AND_NULL() rule
  coccinelle: add a rule to make "type" code use FREE_AND_NULL()
  git-compat-util: add a FREE_AND_NULL() wrapper around free(ptr); ptr = NULL

tree.c

@@ -234,8 +234,7 @@ int parse_tree_gently(struct tree *item, int quiet_on_missing)
 
 void free_tree_buffer(struct tree *tree)
 {
-	free(tree->buffer);
-	tree->buffer = NULL;
+	FREE_AND_NULL(tree->buffer);
 	tree->size = 0;
 	tree->object.parsed = 0;
 }