Add missing null terminator for getopt_long()

getopt_long() assumes an all-zeroes 'struct option' at the end of the
array.  Add it.

Fortunately this isn't causing problems in practice because vold is
always passed valid command line options...

Test: Running 'vold --foo' no longer segfaults.
Change-Id: I2cd3af501cc1aa11327a8062ec492be1d23defdf