ffmpeg: fix segfault with ffmpeg -h

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -4667,7 +4667,7 @@ static const OptionDef options[] = {
     { "async", HAS_ARG | OPT_INT | OPT_EXPERT, {(void*)&audio_sync_method}, "audio sync method", "" },
     { "adrift_threshold", HAS_ARG | OPT_FLOAT | OPT_EXPERT, {(void*)&audio_drift_threshold}, "audio drift threshold", "threshold" },
     { "copyts", OPT_BOOL | OPT_EXPERT, {(void*)&copy_ts}, "copy timestamps" },
-    { "copytb", HAS_ARG | OPT_INT | OPT_EXPERT, {(void*)&copy_tb}, "copy input stream time base when stream copying" },
+    { "copytb", HAS_ARG | OPT_INT | OPT_EXPERT, {(void*)&copy_tb}, "copy input stream time base when stream copying", "source" },
     { "shortest", OPT_BOOL | OPT_EXPERT, {(void*)&opt_shortest}, "finish encoding within shortest input" }, //
     { "dts_delta_threshold", HAS_ARG | OPT_FLOAT | OPT_EXPERT, {(void*)&dts_delta_threshold}, "timestamp discontinuity delta threshold", "threshold" },
     { "xerror", OPT_BOOL, {(void*)&exit_on_error}, "exit on error", "error" },