Don't wait for PID -1

The pid field can be -1 if forking the substituter process failed.

src/libstore/local-store.cc

@@ -358,6 +358,7 @@ LocalStore::~LocalStore()
             i->second.to.close();
             i->second.from.close();
             i->second.error.close();
+            if (i->second.pid != -1)
                 i->second.pid.wait(true);
         }
     } catch (...) {