kconfig: don't crash on NULL expressions in expr_eq()

kconfig: don't crash on NULL expressions in expr_eq()

BugLink: https://bugs.launchpad.net/bugs/1860130
[ Upstream commit 272a72103012862e3a24ea06635253ead0b6e808 ]

NULL expressions are taken to always be true, as implemented by the
expr_is_yes() macro and by several other functions in expr.c. As such,
they ought to be valid inputs to expr_eq(), which compares two
expressions.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>