mesa: Don't reuse DummyFramebuffer as the incomplete framebuffer

Binding framebuffer 0 on a context that doesn't have a winsys drawable
will try to bind the incomplete framebuffer.  That fails when that's
also the dummy framebuffer.
diff --git a/src/mesa/main/context.c b/src/mesa/main/context.c
index 73d96e8..66e4183 100644
--- a/src/mesa/main/context.c
+++ b/src/mesa/main/context.c
@@ -1300,6 +1300,9 @@
    if (ctxvis == bufvis)
       return GL_TRUE;
 
+   if (buffer == _mesa_get_incomplete_framebuffer())
+      return GL_TRUE;
+
 #if 0
    /* disabling this fixes the fgl_glxgears pbuffer demo */
    if (ctxvis->doubleBufferMode && !bufvis->doubleBufferMode)