-Wtautological-undefined-compare warning in tabvector.cpp

Another `-Wtautological-undefined-compare` warning when building for Android using clang and tess-two:

```
jni/com_googlecode_tesseract_android/src/textord/tabvector.cpp:526:7: warning: 'this' pointer cannot be null in well-defined C++ code; comparison may be assumed to always evaluate to false [-Wtautological-undefined-compare]
  if (this == NULL) {
      ^~~~    ~~~~
```
