radeon: Fix compile warnings, compile errors.

src/gallium/winsys/drm/radeon/core/radeon_r300.c

@@ -79,10 +79,15 @@ struct r300_winsys*
 radeon_create_r300_winsys(int fd, struct radeon_winsys* old_winsys)
 {
     struct r300_winsys* winsys = CALLOC_STRUCT(r300_winsys);
+    struct radeon_cs_manager* csm;
+
+    if (winsys == NULL) {
+        return NULL;
+    }
 
     do_ioctls(winsys, fd);
 
-    struct radeon_cs_manager* csm = radeon_cs_manager_gem_ctor(fd);
+    csm = radeon_cs_manager_gem_ctor(fd);
 
     winsys->cs = radeon_cs_create(csm, 1024 * 64 / 4);