Fix streaming plugin rtcp

[sjkummer]

Fix for #2975

[lminiero]

Looking at the code again, I was wondering why this is needed. We're passing sizeof(source->video_rtcp_addr) right now, where video_rtcp_addr is an instance of struct sockaddr_storage, which is larger than both struct sockaddr_in and struct sockaddr_in6, and so is enough to contain them both. As such, I'd expect sendto to only have to check the address family in the shared part to know how to interpret that. I guess something breaks internally if it's not the expected size, though: we do perform something similar to what you added here in RTP forwarders, for instance, and they work as expected, so that's indeed probably needed.

I see this is a patch for 0.x, so I'll port it to master as well. Thanks!