loaders: JPG: Fix read callback
loaders: JPG: Fix read callback

Apparently when I/O read callback returns buffer of size 0 the libjpeg
segfaults. So we now return FALSE from the callback even when read
returns 0, which means end of the file when the underlying I/O is file
based.

Special thanks to the american fuzzy lop (afl).

Signed-off-by: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>

gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim