Assert no duplicate trimming

Ref #33
Ref https://github.com/varnishcache/varnish-cache/pull/4013

src/fellow_cache.c

@@ -4043,6 +4043,10 @@ fellow_busy_obj_getspace(struct fellow_busy *fbo, size_t *sz, uint8_t **ptr)
 	assert(*sz > 0);
 	AN(ptr);
 
+	// #33 https://github.com/varnishcache/varnish-cache/pull/4013
+	if (fbo->body_seglist == NULL)
+		WRONG("_getspace() called after _trimstore()");
+
 	CHECK_OBJ_NOTNULL(fbo->fc, FELLOW_CACHE_MAGIC);
 	CHECK_OBJ_NOTNULL(fbo->fc->tune, STVFE_TUNE_MAGIC);
 	max = (size_t)1 << fbo->fc->tune->chunk_exponent;