We were checking if the attrib_list was NULL to verify that no
unsupported attributes are provides, but a single EGL_NONE attribute
should also be allowed.

Also use operator& to avoid confusing implicit conversion to a pointer.

Change-Id: I3c30968fddfd4d1dfbdff9c4ff291abb258e304c
Reviewed-on: https://swiftshader-review.googlesource.com/20148Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>