Catch SIGBUS in HeapWalker

SIGBUS shouldn't happen, since all of the regions being walked
were previously read out of /prod/pid/maps, but it seems to happen
rarely.  Catch it so it can produce a better log message.

Bug: 128983715
Test: atest memunreachable_test
Change-Id: I82d3941a382a0451c0dda12f5eb849ad8d697bc9
diff --git a/libmemunreachable/HeapWalker.cpp b/libmemunreachable/HeapWalker.cpp
index e11f079..7cae048 100644
--- a/libmemunreachable/HeapWalker.cpp
+++ b/libmemunreachable/HeapWalker.cpp
@@ -207,6 +207,6 @@
   }
 }
 
-ScopedSignalHandler::SignalFn ScopedSignalHandler::handler_;
+Allocator<ScopedSignalHandler::SignalFnMap>::unique_ptr ScopedSignalHandler::handler_map_;
 
 }  // namespace android