glx/dri: Initialize api even if num_attribs == 0.

glx/dri: Initialize api even if num_attribs == 0.

Both dri2_create_context_attribs and drisw_create_context_attribs call
dri2_convert_glx_attribs, expecting it to fill in *api on success.

However, when num_attribs == 0, it was returning true without setting
*api, causing the caller to use an uninitialized value.

Tested-by: Vadim Girlin <vadimgirlin@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>