c++: more YY_NULL

Caught by maintainer-check-g++.
* data/glr.c, data/lalr1.cc, data/yacc.c, tests/cxx-type.at,
* tests/glr-regression.at, tests/push.at:
When simple to do, avoid expliciting the null ptr.
Otherwise use YY_NULL.
(cherry picked from commit eeaf1dc646)

Conflicts:

	data/glr.c
	data/lalr1.cc
	tests/cxx-type.at

tests/glr-regression.at

@@ -298,7 +298,7 @@ static void yyerror(char const * s) {
   fprintf(stderr,"error: %s\n",s);
 }
 
-FILE *input = NULL;
+FILE *input = YY_NULL;
 
 int P[] = { P1, P2 };
 int O[] = { O1, O2 };