Fix 'zdb -o' segmentation fault

Fix a regression accidentally introduced by OpenZFS 7280 in ed828c0: since
whether to accept NULL as a valid first parameter in strchr() is implementation
specific we add an additional check to avoid crashing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #5917