bswap: Fix up a thinko with empty CONSTRUCTORs [PR98378]

The code I've added recently in find_bswap_or_nop for VECTOR CONSTRUCTORs
is missing punt on an important case - namely empty CONSTRUCTORs, because in that
case the loop will not initialize *n and the code after the loop will then
use the uninitialized structure.

2020-12-19  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98378
	* gimple-ssa-store-merging.c (find_bswap_or_nop): Punt if CONSTRUCTOR
	has no elements.