[Ada] Raise exception on call to Expect for a dead process

[Pierre-Marie de Rodat]

Call to Expect for a dead process results in SIGBUS signal on Linux
systems. Process_Died exception is raised in this case now.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-09-18  Vadim Godunko  <godunko@adacore.com>

gcc/ada/

	* libgnat/g-expect.adb (Expect_Internal): Don't include invalid
	file descriptors into the set of file descriptors for Poll.
	Raise Process_Died exception when computed set of file
	descriptors to monitor is empty.

gcc/testsuite/

	* gnat.dg/expect4.adb: New testcase.
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