Crash/internal assertion on usage of unknown define

[povik]

Usage of an unknown define can crash the tool and trigger an internal assertion.

Example input:

module top;
	wire foo = 3 < `BAR;
endmodule

The AST contains InvalidExpression but gets passed to PopulateNetlist anyway. For some reason compilation->hasIssuedErrors() and/or driver.reportDiagnostics(/* quiet */ false) doesn't catch this.

[povik]

Fixed in 9f5b5f4