PJHIGH: fix signed/unsigned mismatches.
PJHIGH: fix casts.
PJHIGH: fix incorrect reference.
PJHIGH: fix picfile.fd.

Picfile.fd should be a Jfile or equivalently a Jfl *, not a Jfile *.
PJHIGH: suppress uninitialised 'bpr' warning.

chunk_type must be PIC_BITPIXELS or PIC_BYTEPIXELS.
PJHIGH: suppress uninitialised 'dif' warning.

dif is only used when usedc >= 0, so dif cannot be uninitialised.
PJHIGH: fix uninitialised 'err' warnings.
PJHIGH: fix uninitialised 'mh' when get_formatted_ftext fails.