[Ada] Fix wrong assumption on bounds in GNAT.Encode_String

[Ada] Fix wrong assumption on bounds in GNAT.Encode_String

This fixes a couple of oversights in the GNAT.Encode_String package,
whose effect is to assume that all the strings have a lower bound of 1.

2019-07-22  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* libgnat/g-encstr.adb (Encode_Wide_String): Fix oversight.
	(Encode_Wide_Wide_String): Likewise.

gcc/testsuite/

	* gnat.dg/encode_string1.adb, gnat.dg/encode_string1_pkg.adb,
	gnat.dg/encode_string1_pkg.ads: New testcase.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@273674 138bc75d-0d04-0410-961f-82ee72b054a4