Fix missing return when trying to skip threads dump when mutator lock is exclusively held

Bug: 134037466
Bug: 134167395
Test: m
Test: m test-art-host
Change-Id: I0b2021f03f7744b580cda62550ee16d39f1bf7e0
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index 28ac65e..ad8ded4 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -565,6 +565,7 @@
         bool ml_already_exlusively_held = Locks::mutator_lock_->IsExclusiveHeld(self);
         if (ml_already_exlusively_held) {
           os << "Skipping all-threads dump as mutator lock is exclusively held.";
+          return;
         }
         bool ml_already_held = Locks::mutator_lock_->IsSharedHeld(self);
         if (!ml_already_held) {