diff_parse: free object instead of its pointer

In e7330016 (diff_parse: check return value of `git_diff_init_options`,
2017-03-20), we've introduced an error check whether we're able to
correctly initialize the diff options. This simple commit actually
introduced a segfault in that we now try to free the pointer to the
allocated diff in an error case, instead of the allocated diff itself.
This commit fixes the issue.

src/diff_parse.c

@@ -45,7 +45,7 @@ static git_diff_parsed *diff_parsed_alloc(void)
 	diff->base.free_fn = diff_parsed_free;
 
 	if (git_diff_init_options(&diff->base.opts, GIT_DIFF_OPTIONS_VERSION) < 0) {
-		git__free(&diff);
+		git__free(diff);
 		return NULL;
 	}