fftools/ffmpeg: handle -enc_time_base -1 during stream creation

There is no reason to postpone it until opening the encoder. Also, abort
when the input stream is unknown, rather than disregard an explicit
request from the user.

fftools/ffmpeg_mux_init.c

@@ -1149,6 +1149,15 @@ static OutputStream *ost_add(Muxer *mux, const OptionsContext *o,
                 av_log(ost, AV_LOG_FATAL, "Invalid time base: %s\n", enc_time_base);
                 exit_program(1);
             }
+            if (q.num < 0) {
+                if (!ost->ist) {
+                    av_log(ost, AV_LOG_FATAL,
+                           "Cannot use input stream timebase for encoding - "
+                           "no input stream available\n");
+                    exit_program(1);
+                }
+                q = ost->ist->st->time_base;
+            }
             ost->enc_timebase = q;
         }
     } else {