Add check to inst visitor

Just tripped on this, a malformed instruction region could make you
endlessly loopy.

Change-Id: Ib65e147c4cd8fe02328bc62aee2bbecfde604455
diff --git a/src/dex_instruction_visitor.h b/src/dex_instruction_visitor.h
index 1a8325c..172fc4f 100644
--- a/src/dex_instruction_visitor.h
+++ b/src/dex_instruction_visitor.h
@@ -29,6 +29,7 @@
           CHECK(true);
       }
       ptr += inst->Size();
+      CHECK_LE(ptr, end);
     }
   }