[BUG] Fixed default reject reason for a listener callback and wrong handling of a listener's reject response

[ethouris]

Fixes #2559

1. Setting the SRT_REJX_FALLBACK value in the listener callback in order to give it a default value for the case of rejection request, and clearing this value in case when the rejection reason was mistakenly set by the callback, but it resulted in acceptance.
2. Fixed the bug in the caller's handling of the listener's rejection callback - value exactly equal to URQ_FAILURE was not treated as error.