mempool didn't work for sizes less than sizeof(void*)

lib/isc/mem.c

@@ -1841,6 +1841,12 @@ isc__mempool_create(isc_mem_t *mctx0, size_t size, isc_mempool_t **mpctxp) {
 	mpctx->common.magic = ISCAPI_MPOOL_MAGIC;
 	mpctx->lock = NULL;
 	mpctx->mctx = mctx;
+	/*
+	 * Mempools are stored as a linked list of element.
+	 */
+	if (size < sizeof(element)) {
+		size = sizeof(element);
+	}
 	mpctx->size = size;
 	mpctx->maxalloc = UINT_MAX;
 	mpctx->allocated = 0;

lib/isc/tests/mem_test.c

@@ -137,6 +137,21 @@ ATF_TC_BODY(isc_mem, tc) {
 
 	isc_mem_destroy(&localmctx);
 
+	result = isc_mem_createx2(0, 0, default_memalloc, default_memfree,
+				  NULL, &localmctx,
+				  ISC_MEMFLAG_FILL | ISC_MEMFLAG_INTERNAL);
+	ATF_REQUIRE_EQ(result, ISC_R_SUCCESS);
+
+	result = isc_mempool_create(localmctx, 2, &mp1);
+	ATF_REQUIRE_EQ(result, ISC_R_SUCCESS);
+
+	tmp = isc_mempool_get(mp1);
+	ATF_CHECK(tmp != NULL);
+
+	isc_mempool_put(mp1, tmp);
+
+	isc_mempool_destroy(&mp1);
+
 	isc_test_end();
 }