This patch fixes a bug in Put_Scaled, which causes a crash when checks
are on.

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

2019-08-12  Bob Duff  <duff@adacore.com>

gcc/ada/

	* libgnat/a-tifiio.adb (Put_Scaled): Prevent AA from being
	negative, since Field is range 0 .. something.