fftools/cmdutils: Fix break command dump for -map option

Before this fix, ffmpeg -h full | grep map get the command dump
like:

-map [-]input_file_id[:stream_specifier][,sync_file_id[:stream_s  set input stream mapping
                                                               ^
                                                               |
							    truncated
after this fix, we can get full option dump.
Found-by: vacingfang <vacingfang@tencent.com>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

fftools/cmdutils.c

@@ -182,7 +182,7 @@ void show_help_options(const OptionDef *options, const char *msg, int req_flags,
 
     first = 1;
     for (po = options; po->name; po++) {
-        char buf[64];
+        char buf[128];
 
         if (((po->flags & req_flags) != req_flags) ||
             (alt_flags && !(po->flags & alt_flags)) ||