c++: Fix crash with broken deduction from {} [PR97895]

Unfortunately, the otherwise beautiful

  for (constructor_elt &elt : *CONSTRUCTOR_ELTS (init))

is not immune to an empty constructor, so we have to check
CONSTRUCTOR_ELTS first.

gcc/cp/ChangeLog:

	PR c++/97895
	* pt.c (do_auto_deduction): Don't crash when the constructor has
	zero elements.

gcc/testsuite/ChangeLog:

	PR c++/97895
	* g++.dg/cpp0x/auto54.C: New test.