Memory corruption in FLV wrapping if there is not enough allocated space for metadata

[lalinsky]

We had a crash when station was using long name. This is the situation:

• You have a relay ICY stream with long stream name (e.g. >180chars, <195 chars).
• You have a client requesting FLV wrapping.
• The client connects to the stream before ICY metadata is sent, so the flvmeta allocation happens in flv_write_metadata where it's limited to 200 bytes.
• It manages to add the "name" field to the flvmeta object and then maybe some small bool fields.
• For anything larger there isn't enough space, but each time there isn't enough space, it appends "\x00\x00\x09" to the meta buffer without checking the size.
• When this happens enough times, it overwrites the allocated space.

In the log file, it looks like this:

[2021-02-17  08:37:58] DBUG flv/flv_meta_increase 4 array elements
[2021-02-17  08:37:58] DBUG flv/flv_meta_increase 4 array elements
[2021-02-17  08:37:58] DBUG flv/flv_meta_increase 4 array elements
[2021-02-17  08:37:58] DBUG flv/flv_meta_increase 4 array elements
[2021-02-17  08:37:58] DBUG flv/flv_meta_increase 4 array elements
[2021-02-17  08:37:58] DBUG flv/flv_meta_increase 4 array elements
[2021-02-17  08:37:58] DBUG flv/flv_meta_increase 4 array elements

Then there is some other activity and then it crashes, because of the corrupted memory.

The fix does two things:

• Makes sure that "\x00\x00\x09" is appended to the flvmeta only once, and only if there is enough space.
• Allocates 4000 bytes in flv_write_metadata, just like it's done in mp3_set_title.

With this patch (and assuming the stream name is really super long, but below the 4000 - 5 bytes), the error log looks like this:

[2021-02-17  08:48:41] WARN flv/flv_meta_append_number not enough space for audiodatarate
[2021-02-17  08:48:41] WARN flv/flv_meta_append_number not enough space for audiosamplerate
[2021-02-17  08:48:41] WARN flv/flv_meta_append_bool not enough space for canSeekToEnd
[2021-02-17  08:48:41] WARN flv/flv_meta_append_bool not enough space for hasMetadata
[2021-02-17  08:48:41] WARN flv/flv_meta_append_bool not enough space for hasVideo
[2021-02-17  08:48:41] WARN flv/flv_meta_append_bool not enough space for hasAudio