remove unnecessary branch in string serialization

Author: grisumbras

include/boost/json/impl/serializer.ipp

@@ -310,10 +310,8 @@ do_str3:
     return w.suspend(writer::state::str3);
 
 do_esc1:
-    if(BOOST_JSON_LIKELY(ss))
-        ss.append(w.buf_[0]);
-    else
-        return w.suspend(writer::state::esc1);
+    BOOST_ASSERT(ss);
+    ss.append(w.buf_[0]);
     goto do_str3;
 
 do_utf1: