Tools/MeshExtractor: Fix uninitialized values

Fix uninitialized struct padding bytes written to output files. These bytes are actually ignored when loading these files, so the main point of this fix is to shut up the Valgrind log.

Valgrind log:
 Syscall param write(buf) points to uninitialised byte(s)
  at : ??? (syscall-template.S:81)
  by : _IO_file_write@@GLIBC_2.2.5 (fileops.c:1270)
  by : new_do_write (fileops.c:546)
  by : _IO_do_write@@GLIBC_2.2.5 (fileops.c:519)
  by : _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1341)
  by : fwrite (iofwrite.c:43)
  by  BuilderThread::svc() (ContinentBuilder.cpp:56)
  by : ACE_Task_Base::svc_run(void*) (in /usr/lib/libACE-6.0.3.so)
  by : ACE_Thread_Adapter::invoke_i() (in /usr/lib/libACE-6.0.3.so)
  by : ACE_Thread_Adapter::invoke() (in /usr/lib/libACE-6.0.3.so)
  by : start_thread (pthread_create.c:311)
  by : clone (clone.S:113)
 Address 0x4039011 is not stack'd, malloc'd or (recently) free'd
 Uninitialised value was created by a stack allocation
  at: BuilderThread::svc() (ContinentBuilder.cpp:30)