Fix unterminated string for the long output file name

The strncpy function is not zero-terminating the target string if all n
bytes were used in the target buffer. That resulted in random bytes
being appended to the end of the buffer when name length exceeded the
length of the buffer.
Remove warnings about uninitialized variables

Compiler was complaining about uninitialized variables:
fs_visualize.c: In function ‘writeImage’:
fs_visualize.c:105:23: warning: ‘row’ may be used uninitialized in this function [-Wmaybe-uninitialized]
fs_visualize.c:103:37: warning: ‘info_ptr’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Variables initialized properly now, compiler is happy.
Cast offset to the value type expected by fprintf

Compiler was complaining about the incorrect value type:
fs_visualize.c: In function ‘main’:
fs_visualize.c:158:3: warning: format ‘%lld’ expects argument of type ‘long long int’, but argument 3 has type ‘off_t’ [-Wformat]

Type cast added, compiler is happy now.