jffsX-utils: jffs2reader: Fix potential null pointer dereference compiling warning...

jffsX-utils: jffs2reader: Fix potential null pointer dereference compiling warning for 'path'

The compiler compains following message:
 jffsX-utils/jffs2reader.c:697:6: warning: potential null pointer
 dereference
It won't bring any problems because the 'path' is guaranteed to be a
non-NULL variable. Fix the warning by adding a NULL pointer check.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>