rtems-tld.cpp: Logically dead code error spotted by Coverity

Original author: rlong

CID 1399595: Logically dead code in generate_traces().

1263            if (sig.has_ret () && !generator_.ret_trace.empty ())
1264            {
1265              if (ds_added)
1266                ds += " + ";
1267              else
1268                ds_added = true;
    const: At condition drs_added, the value of drs_added must be equal to 0.
    dead_error_condition: The condition drs_added cannot be true.
1269              if (drs_added)
    CID 1399595 (#1 of 1): Logically dead code (DEADCODE)dead_error_line: Execution cannot reach this statement: drs += " + ";.
1270                drs += " + ";
1271              else
1272                drs_added = true;
1273              ds += "sizeof(" + sig.ret + ')';
1274              drs += "sizeof(" + sig.ret + ')';
1275            }