ART: Fix ImageSpace error message.

Use *error_msg instead of error_msg.

(cherry picked from commit 50f2e9a6b935ba1c456a687e5f0822d9356d71e5)

Bug: 18143314
Change-Id: I62f6aeda142235eb1a412a4c1ae1104b813e6967
diff --git a/runtime/gc/space/image_space.cc b/runtime/gc/space/image_space.cc
index cd5e196..b67257f 100644
--- a/runtime/gc/space/image_space.cc
+++ b/runtime/gc/space/image_space.cc
@@ -497,7 +497,7 @@
     if (relocated_version_used) {
       LOG(FATAL) << "Attempted to use relocated version of " << image_location << " "
                  << "at " << cache_filename << " generated from " << system_filename << " "
-                 << "but image failed to load: " << error_msg;
+                 << "but image failed to load: " << *error_msg;
       return nullptr;
     } else if (is_system) {
       *error_msg = StringPrintf("Failed to load /system image '%s': %s",