sequencer.c: fix overflow & segfault in parse_strategy_opts()

The split_cmdline() function introduced in [1] returns an "int". If it's negative it signifies an error. The option parsing in [2] didn't account for this, and assigned the value directly to the "size_t xopts_nr". We'd then attempt to loop over all of these elements, and access uninitialized memory. There's a few things that use this for option parsing, but one way to trigger it is with a bad value to "-X <strategy-option>", e.g: git rebase -X"bad argument\"" In another context this might be a security issue, but in this case someone who's already able to inject arguments directly to our commands would be past other defenses, making this potential escalation a moot point. As the example above & test case shows the error reporting leaves something to be desired. The function will loop over the whitespace-split values, but when it encounters an error we'll only report the first element, which is OK, not the second "argument\"" whose quote is unbalanced. This is an inherent limitation of the current API, and the issue affects other API users. Let's not attempt to fix that now. If and when that happens these tests will need to be adjusted to assert the new output. 1. 2b11e3170e9 (If you have a config containing something like this:, 2006-06-05) 2. ca6c6b45dd9 (sequencer (rebase -i): respect strategy/strategy_opts settings, 2017-01-02) Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>