Bug fix on FilterGraph::filter, nb_filters means the count of the filters, idx should less than the count of the filters. (#134)

Author: wooshifu

src/filters/filtergraph.cpp

@@ -70,7 +70,7 @@ FilterContext FilterGraph::filter(unsigned idx, OptionalErrorCode ec)
         return FilterContext();
     }
 
-    if (idx < m_raw->nb_filters) {
+    if (idx >= m_raw->nb_filters) {
         throws_if(ec, Errors::OutOfRange);
         return FilterContext();
     }