Return an invalid StackMap when one cannot be found.

This avoids aborting when handling a crash.

Change-Id: Ie5b5d48061fa9258b349b0284f7b00c5855d9fbd
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 2145c9c..9b1600f 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -2248,6 +2248,7 @@
         uintptr_t native_pc_offset = m->NativeQuickPcOffset(GetCurrentQuickFramePc(), entry_point);
         CodeInfo code_info = m->GetOptimizedCodeInfo();
         StackMap map = code_info.GetStackMapForNativePcOffset(native_pc_offset);
+        DCHECK(map.IsValid());
         MemoryRegion mask = map.GetStackMask(code_info);
         // Visit stack entries that hold pointers.
         for (size_t i = 0; i < mask.size_in_bits(); ++i) {