Fix segfault during delay slot scheduling pass

This is a small regression present on the mainline and 13 branch, although
the underlying problem has probably been there for ages, in the form of a
segfault during the delay slot scheduling pass, for a function that falls
through to exit without any instruction generated for the end of function.

gcc/
	PR rtl-optimization/113140
	* reorg.cc (fill_slots_from_thread): If we are to branch after the
	last instruction of the function, create an end label.

gcc/testsuite/
	* g++.dg/opt/delay-slot-2.C: New test.