#27 Check return codes everywhere

[Markus Elfring]

Some checks for return codes are missing.

Examples:
Would you like to add more error handling for return values from well-known functions like at the following places?

• http://downloads.sourceforge.net/pmt/b2p152s.zip
  BMP2PNG.C:
  fprintf => usage
  fclose => read_file

• http://downloads.sourceforge.net/pmt/pngcrush-1.6.4-nolib.tar.bz2
  pngcrush.c:
  malloc => main

• http://downloads.sourceforge.net/pmt/pngmeta-1.11.tar.gz
  pngmeta.c:
  fputs => print_init