Fix DEX zip open error message printing

We were printing the pointer to the error message string rather than the
string itself.

Bug: none
Test: test-art-host
Change-Id: I06bd9afd100c425039847c52df1f4912966053b8
diff --git a/runtime/dex_file.cc b/runtime/dex_file.cc
index f59420d..d93cbbb 100644
--- a/runtime/dex_file.cc
+++ b/runtime/dex_file.cc
@@ -415,7 +415,7 @@
                                                                          &error_code));
       if (next_dex_file.get() == nullptr) {
         if (error_code != ZipOpenErrorCode::kEntryNotFound) {
-          LOG(WARNING) << error_msg;
+          LOG(WARNING) << "Zip open failed: " << *error_msg;
         }
         break;
       } else {