If there are compilation errors then gigi doesn't backannotate the AST
with sizes, alignment, etc. The postponed compilation validation checks
can then easily crash or give spurious errors. We now just skip them.

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

gcc/ada/

	* gnat1drv.adb (Gnat1drv): Skip postponed checks when there are
	errors.