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

Change-Id: I8c994d1e6a8d2ebe52eaa4f0132e0deb2ecfa5f3
diff --git a/src/runtime_test.cc b/src/runtime_test.cc
index 213897f..a926a23 100644
--- a/src/runtime_test.cc
+++ b/src/runtime_test.cc
@@ -15,7 +15,7 @@
   void* test_exit = reinterpret_cast<void*>(0xc);
   void* null = reinterpret_cast<void*>(NULL);
 
-  std::string lib_core = GetLibCoreDexFileName();
+  std::string lib_core(GetLibCoreDexFileName());
 
   std::string boot_class_path;
   boot_class_path += "-Xbootclasspath:";