avopt: Check whether the object actually has got an AVClass

AVIOContext has got an av_class member that only gets set if
opening the context using avio_open2, but not if allocating a
custom IO context. A caller that wants to read AVOptions from
an AVIOContext (recursively using AV_OPT_SEARCH_CHILDREN) may
not know if the AVIOContext actually has got a class set or not.

Signed-off-by: Martin Storsjö <martin@martin.st>