Silence coverity report.

Coverity: "Unchecked return value (CHECKED_RETURN) check_return: Calling function "ChannelBase::IsInputAvailable(int, uint &) const" without checking return value (as is done elsewhere 5 out of 6 times)"
However, IsInputAvailable is here only used to calculate plexid_restriction

Coverity #1028692

mythtv/libs/libmythtv/recorders/channelbase.cpp

@@ -248,7 +248,7 @@ uint ChannelBase::GetNextChannel(uint chanid, ChannelChangeDirection direction)
     }
 
     uint mplexid_restriction = 0;
-    IsInputAvailable(m_currentInputID, mplexid_restriction);
+    (void)IsInputAvailable(m_currentInputID, mplexid_restriction);
 
     return ChannelUtil::GetNextChannel(
         m_allchannels, chanid, mplexid_restriction, direction);