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

• Related issues: #1516 (closed)
• Requested reviewers: @kiene

Reason why this change is needed

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

Description of the change

Replace sizeof (char *) with strlen().

Affected operating points

Purely BE change. Closes #1516 (closed)