Change in pnmtoimage if image->comps[].data is NULL

[szukw000]

Cnf. #1152.
Correction in opj_decompress.c for input filename without extension.

[szukw000]

@rouault ,
this PR handles two cases:

1. where image->comps[].data is NULL.
2. where the input filename does not have an extension, e.g. '.j2k' etc .
   The old code simply returns the last 4 signs.
   The case where the filename is shorter than 4 signs is not handled.
   I suppose all opj_compress/opj_decompress functions must be reconsidered.
   winfried

[rouault]

Indentation issue reported by https://travis-ci.org/uclouvain/openjpeg/jobs/440595756