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

main.cpp

@@ -152,6 +152,7 @@ static void parse_args(int argc, char** argv) {
         {"blkid_untrusted_context", required_argument, 0, 'B'},
         {"fsck_context", required_argument, 0, 'f'},
         {"fsck_untrusted_context", required_argument, 0, 'F'},
+        {nullptr, 0, nullptr, 0},
     };
 
     int c;