fix: prevent segfault in mjCModel::ResolvePlugin when instance is missing

[ebms03]

When looking up a plugin instance name that does not exist,

<plugin plugin="mujoco.pid">
  <instance name="my_pid_config" /> 
</plugin>
...
<actuator>
  <plugin instance="pid_config" ... /> <!-- wrong name -->
</actuator>

there is a null check in mjCModel::RegisterPlugin for throwing an error if the instance can't be found,

void mjCModel::ResolvePlugin(..., mjCPlugin** plugin_instance) {
    ...
    (*plugin_instance)->plugin_slot = plugin_slot;
    if (!*plugin_instance) {

but the pointer is dereferenced before the the null check. Moving the plugin slot assignment to after this check fixes it. The added test fails due to a segfault before the change, but passes afterwards.