[Ada] Fix segfault in NRV on pathological case

[Eric Botcazou]

This is a regression present on all active branches where the NRV pass 
implemented in gigi segfaults on a pathological case.

Tested on x86_64-suse-linux, applied on all active branches.


2017-02-24  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/trans.c (return_value_ok_for_nrv_p): Add sanity check


2017-02-24  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/opt63.adb: New test.

-- 
Eric Botcazou
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