[Ada] Add adequate guard before calling First_Rep_Item

[Pierre-Marie de Rodat]

New contracts on Ada.Strings.Bounded revealed an unprotected call to
First_Rep_Item on a possibly empty node.

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

gcc/ada/

	* sem_ch13.adb (Build_Predicate_Functions): Add guard.
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