ui-blob: don't segfault when no path is given

It it possible to inspect blobs by specifying only the SHA-1, and CGit
provides links to do so, for example if a tag points directly at a blob.
In this case the path_items structure is never used, but creating it
still causes strlen to be run on a null pointer.  Fix this.

This error was introduced by commit c1633c6 (Update git to v1.7.6.5 -
2013-03-02).

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-blob.c

@@ -80,7 +80,7 @@ void cgit_print_blob(const char *hex, char *path, const char *head)
 	struct commit *commit;
 	struct pathspec_item path_items = {
 		.match = path,
-		.len = strlen(path)
+		.len = path ? strlen(path) : 0
 	};
 	struct pathspec paths = {
 		.nr = 1,