Detect errors when dereferencing an undeclared variable.

Detect errors when dereferencing an undeclared variable.

Attempting to dereference an undeclared variable will cause an ICE.
Also attempting to pass an undeclared variable as an array of type
will also cause an ICE.  This patch detects both conditions and
generates an appropriate error.

gcc/m2/ChangeLog:

	* gm2-compiler/M2Quads.mod (AssignUnboundedVar): Check Type
	against NulSym and call MetaErrorT1 if necessary.
	(AssignUnboundedNonVar): Check Type against NulSym and
	call MetaErrorT1 if necessary.
	(BuildDesignatorPointer): Check Type1 against NulSym and
	call MetaErrorT1 if necessary.

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