Handle unwinding from SEGV on Windows

PR win32/30255 points out that a call to a NULL function pointer will
leave gdb unable to "bt" on Windows.

I tracked this down to the amd64 windows unwinder.  If we treat this
scenario as if it were a leaf function, unwinding works fine.

I'm not completely sure this patch is the best way.  I considered
having it check for 'pc==0' -- but then I figured this could affect
any inaccessible PC, not just the special 0 value.

No test case because I can't run dejagnu tests on Windows.  I tested
this by hand using the test case in the bug.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30255

gdb/amd64-windows-tdep.c

@@ -1098,13 +1098,14 @@ amd64_windows_frame_cache (frame_info_ptr this_frame, void **this_cache)
   cache->sp = extract_unsigned_integer (buf, 8, byte_order);
   cache->pc = pc;
 
+  /* If we can't find the unwind info, keep trying as though this is a
+     leaf function.  This situation can happen when PC==0, see
+     https://sourceware.org/bugzilla/show_bug.cgi?id=30255.  */
   if (amd64_windows_find_unwind_info (gdbarch, pc, &unwind_info,
 				      &cache->image_base,
 				      &cache->start_rva,
-				      &cache->end_rva))
-    return cache;
-
-  if (unwind_info == 0)
+				      &cache->end_rva)
+      || unwind_info == 0)
     {
       /* Assume a leaf function.  */
       cache->prev_sp = cache->sp + 8;