kconfig: fix infinite loop when expanding a macro at the end of file

A macro placed at the end of a file with no newline causes an infinite
loop.

[Test Kconfig]
  $(info,hello)
  \ No newline at end of file

I realized that flex-provided input() returns 0 instead of EOF when it
reaches the end of a file.

Fixes: 104daea149 ("kconfig: reference environment variables directly and remove 'option env='")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/lexer.l

@@ -303,8 +303,11 @@ static char *expand_token(const char *in, size_t n)
 	new_string();
 	append_string(in, n);
 
-	/* get the whole line because we do not know the end of token. */
-	while ((c = input()) != EOF) {
+	/*
+	 * get the whole line because we do not know the end of token.
+	 * input() returns 0 (not EOF!) when it reachs the end of file.
+	 */
+	while ((c = input()) != 0) {
 		if (c == '\n') {
 			unput(c);
 			break;