vim-patch:8.0.0598: building with gcc 7.1 yields new warnings (#8585)

Problem:    Building with gcc 7.1 yields new warnings.
Solution:   Initialize result. (John Marriott)
9e0f6ec076

src/nvim/ex_docmd.c

@@ -8611,7 +8611,8 @@ eval_vars (
     default:
       // should not happen
       *errormsg = (char_u *)"";
-      return NULL;
+      result = (char_u *)"";    // avoid gcc warning
+      break;
     }
 
     resultlen = STRLEN(result);         /* length of new string */