binderThroughputTest: fail when flag is missing argument

Currently, if the last argument is a flag that requires an argument, the
test will perform an out-of-bounds access. Fix this by returning an
error instead.

Test: ran binderThroughputTest with the given flags
Change-Id: Ida1391f96ccb0482eed2614db99f734b89b49472
Signed-off-by: Alice Ryhl <aliceryhl@google.com>