Fix breakage of referring to table_ in static func

Referencing data member table_ in static functions break certain
platforms. Change it to refer to the declare type void** instead.

src/google/protobuf/map.h

@@ -935,7 +935,7 @@ class Map {
     const hasher& hash_function() const { return *this; }
 
     static size_type max_size() {
-      return static_cast<size_type>(1) << (sizeof(table_) >= 8 ? 60 : 28);
+      return static_cast<size_type>(1) << (sizeof(void**) >= 8 ? 60 : 28);
     }
     size_type size() const { return num_elements_; }
     bool empty() const { return size() == 0; }