glx: Initialize have_back.

Signed-off-by: Brian Paul <brianp@vmware.com>
diff --git a/src/glx/dri2_glx.c b/src/glx/dri2_glx.c
index 86b0a27..1c34448 100644
--- a/src/glx/dri2_glx.c
+++ b/src/glx/dri2_glx.c
@@ -188,6 +188,7 @@
    pdraw->base.psc = psc;
    pdraw->bufferCount = 0;
    pdraw->swap_interval = 1;
+   pdraw->have_back = 0;
 
    DRI2CreateDrawable(psc->dpy, xDrawable);