[gdb] Fix dcheck_stop_handler

Some frames have no name. gdb showed an error in these cases. This CL
avoids this by explicitly handling unnamed frames.

R=ahaas@chromium.org

No-Try: true
Change-Id: Id3918705fbfe66306b5d1c34c3638b67e037aa18
Reviewed-on: https://chromium-review.googlesource.com/c/1382211Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58313}

tools/gdbinit

@@ -147,7 +147,7 @@ def dcheck_stop_handler(event):
         message = frame_message.value(frame).string()
       select_frame = frame.older()
       break
-    if frame.name().startswith('V8_Fatal'):
+    if frame.name() is not None and frame.name().startswith('V8_Fatal'):
       select_frame = frame.older()
     frame = frame.older()