File descriptors equal to 0 is wrongly treated as failure.

At https://code.videolan.org/rist/librist/-/blob/master/src/udp.c#L518 there is wrong check, if socket create failed. Only FDs equal to -1 are failures, currently it doesn't allow FD with value 0, which i am getting.

Of course, there are other places in code with this wrong check, but you most likely will find them better.