cppcheck: (error) Memory leak: vtoc

cppcheck: (error) Memory leak: vtoc

Resolve the reported memory leak by using a dedicated local vptr
variable to store the pointer reported by calloc().  Only assign the
passed **vtoc function argument on success, in all other cases vptr
is freed.

[lib/libefi/rdwr_efi.c:403]: (error) Memory leak: vtoc
[lib/libefi/rdwr_efi.c:422]: (error) Memory leak: vtoc
[lib/libefi/rdwr_efi.c:440]: (error) Memory leak: vtoc
[lib/libefi/rdwr_efi.c:454]: (error) Memory leak: vtoc
[lib/libefi/rdwr_efi.c:470]: (error) Memory leak: vtoc

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