Use NULL instead of 0 for pointers.

fopen(3) will return NULL in case it can't open the STREAM.
The malloc will return a pointer to the allocated memory if successful,
otherwise a NULL pointer is returned.

Also add an extra DEBUG1 to print out the error to open a file.

Reviewed by: ed
Differential Revision: https://svnweb.freebsd.org/changeset/base/297959