tree-optimization/101467 - fix make_temp_ssa_name usage

My previous change to vect_gen_while introduced paths which call
make_temp_ssa_name with a NULL name which isn't supported.  The
following fixes that.

2021-07-16  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101467
	* tree-vect-stmts.c (vect_gen_while): Properly guard
	make_temp_ssa_name usage.