icodec: correctly check avio_read return value

It can read less than the requested amount, in which case buf contains
uninitialized data, causing problems like segmentation faults later on.

Also make sure that image->size is positive, so that it can't match a
negative error code.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
