Check size of path buffer before writing into it

This prevents a buffer overrun that could otherwise be triggered by
creating a file called '.git' with contents

  gitdir: (something really long)

Signed-off-by: Greg Brockman <gdb@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -170,6 +170,8 @@ static int is_git_directory(const char *suspect)
 	char path[PATH_MAX];
 	size_t len = strlen(suspect);
 
+	if (PATH_MAX <= len + strlen("/objects"))
+		die("Too long path: %.*s", 60, suspect);
 	strcpy(path, suspect);
 	if (getenv(DB_ENVIRONMENT)) {
 		if (access(getenv(DB_ENVIRONMENT), X_OK))