MINOR: stconn/applet: Add BUG_ON_HOT() to be sure SE_FL_EOS is never set alone

MINOR: stconn/applet: Add BUG_ON_HOT() to be sure SE_FL_EOS is never set alone

SE_FL_EOS flag must never be set on the SE descriptor without SE_FL_EOI or
SE_FL_ERROR. When a mux or an applet report an end of stream, it must be
able to state if it is the end of input too or if it is an error.

Because all this part was recently refactored, especially the applet part,
it is a bit sensitive. Thus a BUG_ON_HOT() is used and not a BUG_ON().