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

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>

scripts/kconfig/confdata.c

@@ -426,6 +426,8 @@ int conf_read_simple(const char *name, int def)
 		conf_lineno++;
 		sym = NULL;
 		if (line[0] == '#') {
+			if (line[1] != ' ')
+				continue;
 			if (memcmp(line + 2, CONFIG_, strlen(CONFIG_)))
 				continue;
 			p = strchr(line + 2 + strlen(CONFIG_), ' ');