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

Change-Id: I8c994d1e6a8d2ebe52eaa4f0132e0deb2ecfa5f3
diff --git a/src/oat_writer.cc b/src/oat_writer.cc
index f3e78e6..32ba4b1 100644
--- a/src/oat_writer.cc
+++ b/src/oat_writer.cc
@@ -601,7 +601,7 @@
 }
 
 OatWriter::OatDexFile::OatDexFile(const DexFile& dex_file) {
-  const std::string& location = dex_file.GetLocation();
+  const std::string& location(dex_file.GetLocation());
   dex_file_location_size_ = location.size();
   dex_file_location_data_ = reinterpret_cast<const uint8_t*>(location.data());
   dex_file_checksum_ = dex_file.GetHeader().checksum_;