Fix error message logging in `art::gc::space::ImageSpace::LoadBootImage`.

Test: m com.android.art
Change-Id: Icbc926abdd1cdccde92d8e2c7cb595e55b6a06ee
diff --git a/runtime/gc/space/image_space.cc b/runtime/gc/space/image_space.cc
index a718b5c..dd9b2e3 100644
--- a/runtime/gc/space/image_space.cc
+++ b/runtime/gc/space/image_space.cc
@@ -3360,7 +3360,9 @@
   std::ostringstream oss;
   bool first = true;
   for (const auto& msg : error_msgs) {
-    if (!first) {
+    if (first) {
+      first = false;
+    } else {
       oss << "\n    ";
     }
     oss << msg;