[Ada] Segmentation_Fault with Integer'Wide_Wide_Value

[Ada] Segmentation_Fault with Integer'Wide_Wide_Value

This patch updates the routines which produce Wide_String and Wide_Wide_String
from a String to construct a result of the proper maximum size which is later
sliced.

2018-07-16  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* libgnat/s-wchwts.adb (Wide_String_To_String): Use the appropriate
	longest sequence factor. Code clean up.
	(Wide_Wide_String_To_String): Use the appropriate longest sequence
	factor.  Code clean up.

gcc/testsuite/

	* gnat.dg/wide_wide_value1.adb: New testcase.

From-SVN: r262713