branches/5.1: Merge r4031 from branches/5.0:

btr_search_drop_page_hash_when_freed(): Check if buf_page_get_gen()
returns NULL.  The page may have been evicted from the buffer pool
between buf_page_peek_if_search_hashed() and buf_page_get_gen(),
because the buffer pool mutex will be released between these two calls.
(Bug #42279)

rb://82 approved by Heikki Tuuri

btr/btr0sea.c

@@ -1105,12 +1105,20 @@ btr_search_drop_page_hash_when_freed(
 	page = buf_page_get_gen(space, page_no, RW_S_LATCH, NULL,
 				BUF_GET_IF_IN_POOL, __FILE__, __LINE__,
 				&mtr);
+	/* Because the buffer pool mutex was released by
+	buf_page_peek_if_search_hashed(), it is possible that the
+	block was removed from the buffer pool by another thread
+	before buf_page_get_gen() got a chance to acquire the buffer
+	pool mutex again.  Thus, we must check for a NULL return. */
+
+	if (UNIV_LIKELY(page != NULL)) {
 
 #ifdef UNIV_SYNC_DEBUG
 		buf_page_dbg_add_level(page, SYNC_TREE_NODE_FROM_HASH);
 #endif /* UNIV_SYNC_DEBUG */
 
 		btr_search_drop_page_hash_index(page);
+	}
 
 	mtr_commit(&mtr);
 }