Fixed backtrace for apps that never called down to 16-bit code.

debugger/stack.c

diff --git a/debugger/stack.c b/debugger/stack.c
index 5cdff62..157d144 100644
--- a/debugger/stack.c
+++ b/debugger/stack.c
@@ -270,7 +270,8 @@
     
     for (ok = TRUE; ok;) {
         if ((frames[frameno].ss == sw_addr.seg) &&
-	    (frames[frameno].ebp >= sw_addr.off)) {
+            sw_addr.off && (frames[frameno].ebp >= sw_addr.off))
+        {
 	   /* 16<->32 switch...
 	    * yes, I know this is confusing, it gave me a headache too */
 	   if (is16) {