Fix NULL pointer dereference in zdb

Clang's static analyzer complained that we dereference a NULL pointer in
dump_path() if we return 0 when there is an error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14044