ada: Fix internal error on loop parameter specifications

ada: Fix internal error on loop parameter specifications

Originally loop parameter specification only occurred in loops, but now
it also occurs in quantified expressions. This patch guards against
flagging non-loop nodes as null loop statements. This was causing
internal compiler errors that were only visible with switch -gnatdk,
which happens to be default in GNATprove testsuite.

gcc/ada/ChangeLog:

	* sem_ch5.adb (Analyze_Loop_Parameter_Specification): Only set
	flag Is_Null_Loop when loop parameter specification comes from
	a loop and not from a quantified expression.