Fix issue tesseract-ocr#2748

Commit 94d0f77 tried to fix issue tesseract-ocr#2741
but created a new problem.

This commit should fix both old and new issue.

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

src/ccmain/linerec.cpp

@@ -62,7 +62,7 @@ bool Tesseract::TrainLineRecognizer(const STRING& input_imagename,
     return false;
   }
   TrainFromBoxes(boxes, texts, block_list, &images);
-  if (images.NumPages() <= 0) {
+  if (images.PagesSize() == 0) {
     tprintf("Failed to read pages from %s\n", input_imagename.c_str());
     return false;
   }

src/ccstruct/imagedata.h

@@ -233,6 +233,9 @@ class DocumentData {
     std::lock_guard<std::mutex> lock(general_mutex_);
     return total_pages_;
   }
+  size_t PagesSize() const {
+    return pages_.size();
+  }
   int64_t memory_used() const {
     std::lock_guard<std::mutex> lock(general_mutex_);
     return memory_used_;