[Ada] Indexing error when calling GNAT.Regpat.Match

[Ada] Indexing error when calling GNAT.Regpat.Match

This patch corrects an error in the compiler whereby a buffer sizing
error fails to get raised when compiling a regex expression with an
insufficiently sized Pattern_Matcher as the documentation indicated.
This, in turn, could lead to indexing errors when attempting to call
Match with the malformed regex program buffer.

gcc/ada/

	* libgnat/s-regpat.adb, libgnat/s-regpat.ads (Compile): Add a
	new defaulted parameter Error_When_Too_Small to trigger an
	error, if specified true, when Matcher is too small to hold the
	compiled regex program.