BR 3392669: in expand_one_smacro(), we may reach EOL before tafter

If the rest of the line is consumed, we may never see tafter, so we
have to test for end of line at line 5412.  We already do at 5397, so
it clearly should have been there all along.

Reported-by: <puppet@zju.edu.cn>
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

asm/preproc.c

@@ -5409,7 +5409,7 @@ static SMacro *expand_one_smacro(Token ***tpp)
     }
 
     **tpp = tline;
-    for (t = tline; t != tafter; t = t->next)
+    for (t = tline; t && t != tafter; t = t->next)
         *tpp = &t->next;
 
     m->in_progress = false;