patch 9.0.0558: Coverity warns for possibly using NULL pointer

Problem:    Coverity warns for possibly using NULL pointer.
Solution:   Only use "evalarg" when not NULL.

src/eval.c

@@ -6380,7 +6380,8 @@ handle_subscript(
 	    **arg = NUL;
 
 	    idx = find_exported(rettv->vval.v_number, exp_name, &ufunc, &type,
-			    evalarg->eval_cctx, evalarg->eval_cstack, verbose);
+		       evalarg == NULL ? NULL : evalarg->eval_cctx,
+		       evalarg == NULL ? NULL : evalarg->eval_cstack, verbose);
 	    **arg = cc;
 
 	    if (idx < 0 && ufunc == NULL)

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    558,
 /**/
     557,
 /**/