BUG/MEDIUM: stick-table: fix regression caused by a change in proxy struct

In commit 1b8e68e ("MEDIUM: stick-table: Stop handling stick-tables as
proxies."), the ->table member of proxy struct was replaced by a pointer
that is not always checked and in some situations can cause a segfault,
eg. during reload or while using "show table" on CLI socket.

No backport is needed.
diff --git a/src/stick_table.c b/src/stick_table.c
index d7e1eb8..1263ca4 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -3588,7 +3588,8 @@
 		return 1;
 	}
 
-	if (!((struct proxy *)appctx->ctx.table.target)->table->data_ofs[appctx->ctx.table.data_type]) {
+	if (!((struct proxy *)appctx->ctx.table.target)->table ||
+	    !((struct proxy *)appctx->ctx.table.target)->table->data_ofs[appctx->ctx.table.data_type]) {
 		appctx->ctx.cli.severity = LOG_ERR;
 		appctx->ctx.cli.msg = "Data type not stored in this table\n";
 		appctx->st0 = CLI_ST_PRINT;