Missing check for graph manager in JackAPI.

This seems to be the only place where the result of GetGraphManager() is
not checked for null, at least in JackAPI. Found by llvm scan-build.

common/JackAPI.cpp

@@ -510,7 +510,7 @@ LIB_EXPORT int jack_port_tie(jack_port_t* src, jack_port_t* dst)
         jack_error("jack_port_tie called with ports not belonging to the same client");
         return -1;
     } else {
-        return manager->GetPort(mydst)->Tie(mysrc);
+        return (manager ? manager->GetPort(mydst)->Tie(mysrc) : -1);
     }
 }