rmenc: more than 2 streams are not currently possible.

It is possible that just extending the RMMuxContext.streams
array would avoid it.
It is also possible that two audio streams will fail to mux
correctly as well, though at least it should not crash for
this reason.
I do not feel like checking either of these.
This patch fixes trac issue #1022 (at least it makes it
exit with a proper error message instead of crashing).

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
