fix: follow the comments

Signed-off-by: yihong0618 <zouzou0208@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: yihong0618

Lib/pdb.py

@@ -2111,6 +2111,9 @@ def do_list(self, arg):
         exception was originally raised or propagated is indicated by
         ">>", if it differs from the current line.
         """
+        if not self.curframe:
+            self.error('No current frame.')
+            return
         self.lastcmd = 'list'
         last = None
         if arg and arg != '.':
@@ -2134,9 +2137,6 @@ def do_list(self, arg):
             first = self.lineno + 1
         if last is None:
             last = first + 10
-        if not self.curframe:
-            self.error('No current frame.')
-            return
         filename = self.curframe.f_code.co_filename
         breaklist = self.get_file_breaks(filename)
         try: