Fix compiler warning [-Wtautological-undefined-compare]
Fix compiler warning [-Wtautological-undefined-compare]

clang warning:

src/lstm/networkio.cpp:56:15: warning:
 'this' pointer cannot be null in well-defined C++ code;
 comparison may be assumed to always evaluate to true [-Wtautological-undefined-compare]

Signed-off-by: Stefan Weil <8d4c780fcfdc41841e5070f4c43da8958ba6aec0@weilnetz.de>

tesseract-ocr/tesseract,amitdo/tesseract,stweil/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,jbarlow83/tesseract,jbarlow83/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,tesseract-ocr/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,stweil/tesseract,stweil/tesseract,UB-Mannheim/tesseract,stweil/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,jbarlow83/tesseract,stweil/tesseract