c++: Fix up build_zero_init_1 once more [PR99106]

c++: Fix up build_zero_init_1 once more [PR99106]

My earlier build_zero_init_1 patch for flexible array members created
an empty CONSTRUCTOR.  As the following testcase shows, that doesn't work
very well because the middle-end doesn't expect CONSTRUCTOR elements with
incomplete type (that the empty CONSTRUCTOR at the end of outer CONSTRUCTOR
had).

The following patch just doesn't add any CONSTRUCTOR for the flexible array
members, it doesn't seem to be needed.

2021-02-17  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/99106
	* init.c (build_zero_init_1): For flexible array members just return
	NULL_TREE instead of returning empty CONSTRUCTOR with non-complete
	ARRAY_TYPE.

	* g++.dg/ubsan/pr99106.C: New test.

(cherry picked from commit af868e89ec21340d1cafd26eaed356ce4b0104c3)