middle-end: explicitly initialize vec_stmts [PR113132]

when configured with --enable-checking=release we get a false
positive on the use of vec_stmts as the compiler seems unable
to notice it gets initialized through the pass-by-reference.

This explicitly initializes the local.

gcc/ChangeLog:

	PR bootstrap/113132
	* tree-vect-loop.cc (vect_create_epilog_for_reduction): Initialize vec_stmts;

gcc/tree-vect-loop.cc

@@ -6207,7 +6207,7 @@ vect_create_epilog_for_reduction (loop_vec_info loop_vinfo,
   exit_bb = loop_exit->dest;
   exit_gsi = gsi_after_labels (exit_bb);
   reduc_inputs.create (slp_node ? vec_num : ncopies);
-  vec <gimple *> vec_stmts;
+  vec <gimple *> vec_stmts = vNULL;
   for (unsigned i = 0; i < vec_num; i++)
     {
       gimple_seq stmts = NULL;