Don't default-construct std::strings and then assign them.

Change-Id: I8c994d1e6a8d2ebe52eaa4f0132e0deb2ecfa5f3
diff --git a/src/runtime.cc b/src/runtime.cc
index 1304787..ad86e05 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -278,7 +278,7 @@
     } else if (option.starts_with("-Xcheck:jni")) {
       parsed->check_jni_ = true;
     } else if (option.starts_with("-Xrunjdwp:") || option.starts_with("-agentlib:jdwp=")) {
-      std::string tail = option.substr(option[1] == 'X' ? 10 : 15).ToString();
+      std::string tail(option.substr(option[1] == 'X' ? 10 : 15).ToString());
       if (tail == "help" || !Dbg::ParseJdwpOptions(tail)) {
         LOG(FATAL) << "Example: -Xrunjdwp:transport=dt_socket,address=8000,server=y\n"
                    << "Example: -Xrunjdwp:transport=dt_socket,address=localhost:6500,server=n";