[Ada] Assert failure on invalid Variable_Indexing

[Pierre-Marie de Rodat]

GNAT is getting confused and will either generate an assert failure or a
confusing error message (when assertions are disabled) in case of an
invalid Variable_Indexing.

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

gcc/ada/

	* sem_ch4.adb (Try_Container_Indexing): Add ??? comment. Protect
	against malformed tree in case of errors.
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