ren: do not execute tr.c requests when a character is queued in ren.c

Functions like ren_tab() may push back a newline with ren_back().  If
tr_nextreq() is called unconditionally, the newline may be read in the
wrong order (after executing its following request).  The queued
newline may also result in pushing an extra newline with in_push() in
trap_exec().  This patch handles these cases.

Reported by Dirk-Wilhelm Peters <peters@schwertfische>.