tccpp: Fix #ifdef in macro args

see testcase.  We have to "empty" the macro-stack string on
end_macro, as it may be 'tokstr_buf' which is going to be used
twice in recursive invocations of macro_subst_tok.  The uses aren't
overlapping but the first one needs to be properly finalized.

tccpp.c

@@ -1189,6 +1189,7 @@ ST_FUNC void end_macro(void)
     macro_stack = str->prev;
     macro_ptr = str->prev_ptr;
     file->line_num = str->save_line_num;
+    str->len = 0; /* matters if str not alloced, may be tokstr_buf */
     if (str->alloc != 0) {
         if (str->alloc == 2)
             str->str = NULL; /* don't free */

tests/pp/22.c

@@ -0,0 +1,12 @@
+#define TRACE(a,b,c) X a X b X c X
+#define rettrue(x) 1
+A rettrue(bla) B
+TRACE(
+      ARG_1,
+#if rettrue(bla)
+      ARG_2,
+#else
+      ARG_2_wrong,
+#endif
+      ARG_3
+);

tests/pp/22.expect

@@ -0,0 +1,2 @@
+A 1 B
+X ARG_1 X ARG_2 X ARG_3 X;