BUG/MINOR: server: set rid default value in new_server()

BUG/MINOR: server: set rid default value in new_server()

srv->rid default value is set in _srv_parse_init() after the server is
succesfully allocated using new_server().

This is wrong because new_server() can be used independently so rid value
assignment would be skipped in this case.

Hopefully new_server() allocates server data using calloc() so srv->rid
is already set to 0 in practise. But if calloc() is replaced by malloc()
or other memory allocating function that doesn't zero-initialize srv
members, this could lead to rid being uninitialized in some cases.

This should be backported in 2.8 with 61e3894dfe ("MINOR: server: add
srv->rid (revision id) value")