kconfig: require a space after '#' for valid input

[ Upstream commit 4d137ab0 ]

Currently, when an input line starts with '#', (line + 2) is passed to
memcmp() without checking line[1].

It means that line[1] can be any arbitrary character. For example,
"#KCONFIG_FOO is not set" is accepted as valid input, functioning the
same as "# CONFIG_FOO is not set".

More importantly, this can potentially lead to a buffer overrun if
line[1] == '\0'. It occurs if the input only contains '#', as
(line + 2) points to an uninitialized buffer.

Check line[1], and skip the line if it is not a space.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Stable-dep-of: a409fc14 ("kconfig: fix memory leak in sym_warn_unmet_dep()")
Signed-off-by: Sasha Levin <sashal@kernel.org>