Assert dep_stream is non-null to shut up scan-build

lib/nghttp2_session.c

@@ -660,6 +660,8 @@ int nghttp2_session_reprioritize_stream(
     }
   }
 
+  assert(dep_stream);
+
   if (dep_stream == stream->dep_prev && !pri_spec->exclusive) {
     /* This is minor optimization when just weight is changed.
        Currently, we don't reschedule stream in this case, since we