Split rendering RTP dump output file - strlen(needed) instead of sizeof

Clang-tidy complains about a part in the decoder about string operations for a split rendering TRP dump output file:

I think it is correct. If your filename is FILENAME_MAX (1024) chars long, this will only not write out of bounds because sizeof(char *) is 8 (at least on my machine) and ".sr.txt" by chance has 7 characters. Also, I think a null terminator string is missing, but not sure about that.

Proposal: use strlen() instead of sizeof.