cppcheck: (warning) Possible null pointer dereference: nvh

Move the 'nvh = (void *)buf' assignment after the 'buf == NULL'
check to resolve the warning. Interestingly, cppcheck 1.88
correctly determines that the existing code is safe, while
cppcheck 1.86 reports the warning.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9732