PR modula2/127469: ICE when compiling code containing aggregate record using an array field

This patch checks for a NulSym type during WalkConvert PCSymBuild
and ignores it.  The NulSym type presented to BuildConstructor in
M2Quads generates an error message at the typename or '{' if the
typename is absent.  It would be good to spell check the typename
at some future point.

gcc/m2/ChangeLog:

	PR modula2/127469
	* gm2-compiler/M2Quads.mod (BuildConstructor): Check for
	tok UnknownTokenNo and if not equal generate an error message
	at tok stating the type is unknown.
	* gm2-compiler/PCSymBuild.mod (WalkConvert): Check totype for
	NulSym and if so return FALSE.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>