Trimmed vtables of trailing zeroes.

This is something the format supports, but none of the builders
were doing. Can save 10-20% on FlatBuffer binary size!

Also fixed the Go tests.

Change-Id: I616c56ce9bbcfcaee23aa24f0532fcb60b6a8c75
Tested: on Linux.

src/idl_gen_cpp.cpp

@@ -1531,9 +1531,8 @@ class CppGenerator : public BaseGenerator {
              "(const {{STRUCT_NAME}}Builder &);";
 
     // Finish() function.
-    auto num_fields = NumToString(struct_def.fields.vec.size());
     code_ += "  flatbuffers::Offset<{{STRUCT_NAME}}> Finish() {";
-    code_ += "    const auto end = fbb_.EndTable(start_, " + num_fields + ");";
+    code_ += "    const auto end = fbb_.EndTable(start_);";
     code_ += "    auto o = flatbuffers::Offset<{{STRUCT_NAME}}>(end);";
 
     for (auto it = struct_def.fields.vec.begin();

src/idl_parser.cpp

@@ -1071,8 +1071,7 @@ CheckedError Parser::ParseTable(const StructDef &struct_def, std::string *value,
     builder_.PopBytes(struct_def.bytesize);
     assert(!ovalue);
   } else {
-    auto val = builder_.EndTable(start,
-                          static_cast<voffset_t>(struct_def.fields.vec.size()));
+    auto val = builder_.EndTable(start);
     if (ovalue) *ovalue = val;
     if (value) *value = NumToString(val);
   }

src/reflection.cpp

@@ -481,7 +481,7 @@ Offset<const Table *> CopyTable(FlatBufferBuilder &fbb,
     fbb.ClearOffsets();
     return fbb.EndStruct();
   } else {
-    return fbb.EndTable(start, static_cast<voffset_t>(fielddefs->size()));
+    return fbb.EndTable(start);
   }
 }