refactor various if (x) FREE_AND_NULL(x) to just FREE_AND_NULL(x)

Change the few conditional uses of FREE_AND_NULL(x) to be
unconditional. As noted in the standard[1] free(NULL) is perfectly
valid, so we might as well leave this check up to the C library.

1. http://pubs.opengroup.org/onlinepubs/9699919799/functions/free.html

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

blame.c

@@ -313,9 +313,7 @@ static void fill_origin_blob(struct diff_options *opt,
 
 static void drop_origin_blob(struct blame_origin *o)
 {
-	if (o->file.ptr) {
-		FREE_AND_NULL(o->file.ptr);
-	}
+	FREE_AND_NULL(o->file.ptr);
 }
 
 /*

branch.c

@@ -25,9 +25,7 @@ static int find_tracked_branch(struct remote *remote, void *priv)
 			tracking->remote = remote->name;
 		} else {
 			free(tracking->spec.src);
-			if (tracking->src) {
-				FREE_AND_NULL(tracking->src);
-			}
+			FREE_AND_NULL(tracking->src);
 		}
 		tracking->spec.src = NULL;
 	}

http.c

@@ -2418,9 +2418,7 @@ void release_http_object_request(struct http_object_request *freq)
 		close(freq->localfile);
 		freq->localfile = -1;
 	}
-	if (freq->url != NULL) {
-		FREE_AND_NULL(freq->url);
-	}
+	FREE_AND_NULL(freq->url);
 	if (freq->slot != NULL) {
 		freq->slot->callback_func = NULL;
 		freq->slot->callback_data = NULL;

tree-diff.c

@@ -557,9 +557,7 @@ struct combine_diff_path *diff_tree_paths(
 	 * free pre-allocated last element, if any
 	 * (see path_appendnew() for details about why)
 	 */
-	if (p->next) {
-		FREE_AND_NULL(p->next);
-	}
+	FREE_AND_NULL(p->next);
 
 	return p;
 }