preproc: skip invalid advance in %substr

%substr contained a token skip to "skip expanded ID", which is
incorrect, as that has already been skipped at that point. It worked
anyway, accidentally, as this token would always be a whitespace token
-- but we then do skip_white() immediately thereafter.

Delete this to allow this code to be factored.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Author: H. Peter Anvin

asm/preproc.c

@@ -4780,10 +4780,7 @@ static int do_directive(Token *tline, Token **output)
         tline = expand_smacro(tline->next);
         last->next = NULL;
 
-        if (tline) /* skip expanded id */
-            t = tline->next;
-
-        t = skip_white(t);
+        t = skip_white(tline);
 
         /* t should now point to the string */
         if (!tok_is(t, TOKEN_STR)) {