CCBC-1673: do not return block with deallocs as free

Change-Id: I063a84fa9f255918d8a44568c52025bd69429c19
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/232550
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Michael Reiche <michael.reiche@couchbase.com>

Author: avsej

src/netbuf/netbuf.c

@@ -125,7 +125,7 @@ static nb_MBLOCK *find_free_block(nb_MBPOOL *pool, nb_SIZE capacity)
     SLLIST_ITERFOR(&pool->avail, &iter)
     {
         nb_MBLOCK *cur = SLLIST_ITEM(iter.cur, nb_MBLOCK, slnode);
-        if (cur->nalloc >= capacity) {
+        if (cur->nalloc >= capacity && !BLOCK_HAS_DEALLOCS(cur)) {
             sllist_iter_remove(&pool->avail, &iter);
             pool->curblocks--;
             return cur;