ART: Fix ImageSpace error message.

Use *error_msg instead of error_msg.

Change-Id: Idabee3e4a66effd9f6346a2640a3038c5448889e
diff --git a/runtime/gc/space/image_space.cc b/runtime/gc/space/image_space.cc
index d479038..c020240 100644
--- a/runtime/gc/space/image_space.cc
+++ b/runtime/gc/space/image_space.cc
@@ -487,7 +487,7 @@
     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",