job: Fix vimscript wrapper by returning when an invalid id is passed

src/nvim/eval.c

@@ -10535,6 +10535,7 @@ static void f_job_write(typval_T *argvars, typval_T *rettv)
   if (!job) {
     // Invalid job id
     EMSG(_(e_invjob));
+    return;
   }
 
   WBuffer *buf = wstream_new_buffer(xstrdup((char *)argvars[1].vval.v_string),