tests/qtest/libqos/ahci.c: Avoid NULL dereference in ahci_exec()

tests/qtest/libqos/ahci.c: Avoid NULL dereference in ahci_exec()

In ahci_exec() we attempt to permit the caller to pass a NULL pointer
for opts_in (in which case we use a default set of options).  However
although we check for NULL when setting up the opts variable at the
top of the function, we unconditionally dereference opts_in at the
end of the function as part of freeing the opts->buffer.

Switch to checking whether the final buffer is the same as the
buffer we started with, instead of assuming the value we started
with is always opts_in->buffer.

At the moment all the callers pass a non-NULL opts argument, so
we never saw any crashes in practice.

Fixes: Coverity CID 1432302
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20201103115257.23623-1-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>