libproxychains.c: fix NULL pointer dereference after fopen().

if a configuration file can be accessed by proxychains but there is a
mandatory access control (or other) block on the target appication's
ability to read that file, fopen() creates a NULL pointer that will
cause a segfault in fgets().

closes #17

Signed-off-by: mancha <mancha1@hush.com>
Merge pull request #18 from mancha1/master

Fix NULL pointer deref (issue #17)