mesa: fix NULL pointer deref bug in _mesa_drawbuffers()
This is a follow-on fix to commit 39b40ad1. Fixes a crash if the user calls glDrawBuffers(0, NULL). Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82814 Cc: "10.2" <mesa-stable@lists.freedesktop.org> Reviewed-by:Roland Scheidegger <sroland@vmware.com>