Fail in the right place when we cannot allocate a mark stack.

Previously we'd keep going and then crash when dereferencing
the mark stack later. I expected dalvikvm to throw an OutOfMemoryError
in this scenario but it also aborts.

Change-Id: I3cd995113561f40b191fcb51ecfcb1f032bed4ec
diff --git a/src/mark_sweep.h b/src/mark_sweep.h
index 11afd1c..c517246 100644
--- a/src/mark_sweep.h
+++ b/src/mark_sweep.h
@@ -29,7 +29,7 @@
   ~MarkSweep();
 
   // Initializes internal structures.
-  bool Init();
+  void Init();
 
   // Marks the root set at the start of a garbage collection.
   void MarkRoots();