c++: throwing dtor and empty try [PR113088]

c++: throwing dtor and empty try [PR113088]

maybe_splice_retval_cleanup assumed that the function body can't be empty if
there's a throwing cleanup, but when I added cleanups to try blocks in
r12-6333-gb10e031458d541 I didn't adjust that assumption.

	PR c++/113088
	PR c++/33799

gcc/cp/ChangeLog:

	* except.cc (maybe_splice_retval_cleanup): Handle an empty block.

gcc/testsuite/ChangeLog:

	* g++.dg/eh/return2.C: New test.