rev-parse: check i before using argv[i] against argc

rev-parse: check i before using argv[i] against argc

The --prefix, --default, and --resolve-git-dir options to
git-rev-parse require an argument, but when given no argument,
the code uses the NULL read from argv[argc] without checking,
leading to a segfault.

Instead, check first and die() with an error message.

Signed-off-by: David Sharp <dhsharp@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>