radeon: Also flush if it's not the current context that's being destroyed.

This avoids calling radeonFlush() during context destruction, when
ctx->DrawBuffer would be NULL.

NOTE: This is a candidate for the 7.8 branch.

src/mesa/drivers/dri/radeon/radeon_common_context.c

@@ -300,10 +300,10 @@ void radeonDestroyContext(__DRIcontext *driContextPriv )
 	_mesa_meta_free(radeon->glCtx);
 
 	if (radeon == current) {
-		radeon_firevertices(radeon);
 		_mesa_make_current(NULL, NULL, NULL);
 	}
 
+	radeon_firevertices(radeon);
 	if (!is_empty_list(&radeon->dma.reserved)) {
 		rcommonFlushCmdBuf( radeon, __FUNCTION__ );
 	}