Bugs in process_batch_dir()

[stephaniereinders]

I ran process_batch_dir() on the entire CSAFE Handwriting Database and the following errors were returned:

error with document w0239_s01_pLND_r02.png Error in png::readPNG(path): libpng error: Read Error
error with document w0367_s01_pWOZ_r02.png Error in graphs[[i]] <- as.numeric(strs[which(tmp == i)]): attempt to select less than one element in integerOneIndex
error with document w0501_s02_pLND_r03.png Error in graphs[[i]] <- as.numeric(strs[which(tmp == i)]): attempt to select less than one element in integerOneIndex