bstr: fix possible undefined behavior with length 0 strings

BSTR_P() passes the string length and start pointer to printf-like
functions. If the lenfth is 0, the pointer can be NULL, but we're
actually still not allowed to pass a NULL pointer in any case.

This is mostly a technically, because nobody in their right mind would
attempt to specifically break such cases. But it's still undefined
behavior, and some libcs might be strict about this.