src/fill2: fix segfault during fopen error cleanup

src/fill2: fix segfault during fopen error cleanup

When fill2 fails to open the output file (e.g. due to ENOSPC), it jumps
into the cleanup code where it attempts to call fclose, and this causes
a segfault within the glibc fclose code as it attempts to deref a null
pointer.

Fix it by conditionally calling fclose on the file pointer only when
non-null.

This is consistently reproducible with xfs/041.

Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>