skeletons: beware not to use yyarg when it's null

Reported by Adrian Vogelsgesang.

* data/skeletons/glr.c, data/skeletons/lalr1.cc,
* data/skeletons/lalr1.java, data/skeletons/yacc.c: Here.

TODO

@@ -1,7 +1,4 @@
 * Bison 3.6
-** syntax_error_arguments
-Don't store the lookahead if !argv.
-
 ** Documentation
 - yyexpected_tokens in all the languages.
 - YYNOMEM

data/skeletons/glr.c

@@ -2188,7 +2188,9 @@ yy_syntax_error_arguments (const yyGLRStack* yystackp,
   if (yytoken != YYSYMBOL_YYEMPTY)
     {
       int yyn;
-      yyarg[yycount++] = yytoken;
+      if (yyarg)
+        yyarg[yycount] = yytoken;
+      ++yycount;
       yyn = yyexpected_tokens (yystackp, yyarg ? yyarg + 1 : yyarg, yyargn - 1);
       if (yyn == YYNOMEM)
         return YYNOMEM;

data/skeletons/lalr1.cc

@@ -1458,7 +1458,8 @@ b4_dollar_popdef])[]dnl
 
     if (!yyctx.lookahead ().empty ())
       {
-        yyarg[0] = yyctx.token ();
+        if (yyarg)
+          yyarg[0] = yyctx.token ();
         int yyn = yyctx.yyexpected_tokens (yyarg ? yyarg + 1 : yyarg, yyargn - 1);
         return yyn + 1;
       }

data/skeletons/lalr1.java

@@ -981,7 +981,9 @@ b4_dollar_popdef[]dnl
     int yycount = 0;
     if (yyctx.getToken () != null)
       {
-        yyarg[yycount++] = yyctx.getToken ();
+        if (yyarg != null)
+          yyarg[yycount] = yyctx.getToken ();
+        yycount += 1;
         yycount += yyctx.getExpectedTokens (yyarg, 1, yyargn);
       }
     return yycount;

data/skeletons/yacc.c

@@ -1360,7 +1360,9 @@ yy_syntax_error_arguments (const yypcontext_t *yyctx,
     {
       int yyn;]b4_lac_if([[
       YYDPRINTF ((stderr, "Constructing syntax error message\n"));]])[
-      yyarg[yycount++] = yyctx->yytoken;
+      if (yyarg)
+        yyarg[yycount] = yyctx->yytoken;
+      ++yycount;
       yyn = yyexpected_tokens (yyctx, yyarg ? yyarg + 1 : yyarg, yyargn - 1);
       if (yyn == YYNOMEM)
         return YYNOMEM;]b4_lac_if([[