UserspaceEmulator: When auditing accesses, show nearest mallocation

Instead of always showing the preceding mallocation, prefer showing the
following one *if* it's closer to the audited address.

This makes it easier to find bugs where the access is just before an
allocation instead of just after it.

DevTools/UserspaceEmulator/MallocTracer.h

@@ -64,6 +64,7 @@ private:
 
     Mallocation* find_mallocation(FlatPtr);
     Mallocation* find_mallocation_before(FlatPtr);
+    Mallocation* find_mallocation_after(FlatPtr);
     bool is_reachable(const Mallocation&) const;
 
     Vector<Mallocation> m_mallocations;