fread() parameters were in the wrong order

From the fread manpage :
```
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);`

[...] fread() reads nmemb elements of data, each size bytes long [...]
```

I don't know how it could have worked before. With the fix I can use loadJPEG with data_mode==IMAGE_MIME correctly.