Release memory instead of crashing on malformed ESI

When an ESI tag contains an unterminated quote or misses its ending
'>', we release the relevant buffers instead of crashing.

Fixes: #1904

bin/varnishd/cache/cache_esi_parse.c

@@ -1080,8 +1080,11 @@ VEP_Finish(struct vep_state *vep)
 
 	CHECK_OBJ_NOTNULL(vep, VEP_MAGIC);
 
-	AZ(vep->include_src);
-	AZ(vep->attr_vsb);
+	if (vep->include_src)
+		VSB_destroy(&vep->include_src);
+	if (vep->attr_vsb)
+		VSB_destroy(&vep->attr_vsb);
+
 	if (vep->o_pending)
 		vep_mark_common(vep, vep->ver_p, vep->last_mark);
 	if (vep->o_wait > 0) {