fix null pointer dereference (segfault) when Cookie header is empty

Some days we get hundreds of Apache segmentation faults -- other times weeks go by without one.  It turns out that getCASCookie croaks if a client sends an empty Cookie: header.  This patch fixes the problem.  I added a test too.

I think it may be clearer to change this to something like:

```c
        while (cookie != NULL) {
            while (*cookie == ' ') {
                cookie++;
            }
            if (strncmp(cookie, cookieName, strlen(cookieName)) == 0) {
              /* skip to the meat of the parameter (the value after the '=') */
                cookie += (strlen(cookieName)+1);
                rv = apr_pstrdup(r->pool, cookie);
                break;
            }
            cookie = apr_strtok(NULL, ";", &tokenizerCtx);
        }
```

It's mostly a matter of style, but that reads easier to me.