ada: Avoid repeated range checks when negating a rational number

ada: Avoid repeated range checks when negating a rational number

Use local constant to avoid repeated range checks (at least in the debug
builds), but also to make the code easier to read and consistent in style
with similar routines in the same package.

gcc/ada/ChangeLog:

	* urealp.adb (UR_Negate): Capture array element in a local constant.