Check stat for error before setting permissions.

When setting wallpapers without --no-fehbg option, a ~/.fehbg file is
created. This file is set to be an executable for later re-use.

Calling stat() without checking the return value can lead to issues.
If the call fails, then s.st_mode is undefined and excessive permissions
could be set to .fehbg, at worst even setuid/setgid bits for a world
writable file.

While adjusting this, I changed the code to use fstat() and fchmod() to
avoid a further -- but very unlikely -- issue: race condition in form of
TOCTOU. If the file ~/.fehsetbg is replaced by a symlink right before
the chmod call, then a different file would be set executable + the
default mode of the (newly created) file. I don't expect this to be a
real world issue but changed this part "while at it" anyway for more
robust code and a good example on how to handle files.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>