zsmalloc: zs_destroy_pool: add size_class NULL check

Inside the zs_destroy_pool() function, there can still be NULL size_class
pointers: if when the next size_class is allocated, inside
zs_create_pool() function, kzalloc will return NULL and handling the error
condition, zs_create_pool() will call zs_destroy_pool().

Link: https://lkml.kernel.org/r/20221013112825.61869-1-avromanov@sberdevices.ru
Fixes: f24263a5 ("zsmalloc: remove unnecessary size_class NULL check")
Signed-off-by: Alexey Romanov <avromanov@sberdevices.ru>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zsmalloc.c

@@ -2311,6 +2311,9 @@ void zs_destroy_pool(struct zs_pool *pool)
 		int fg;
 		struct size_class *class = pool->size_class[i];
 
+		if (!class)
+			continue;
+
 		if (class->index != i)
 			continue;