gh-138641: prevent accessing non-existing curframe in pdb commands

Lib/pdb.py

@@ -1332,6 +1332,9 @@ def do_break(self, arg, temporary=False):
                     if bp:
                         self.message(bp.bpformat())
             return
+        if not self.curframe:
+            self.error('No current frame.')
+            return
         # parse arguments; comma has lowest precedence
         # and cannot occur in filename
         filename = None
@@ -1411,6 +1414,7 @@ def do_break(self, arg, temporary=False):
     # To be overridden in derived debuggers
     def defaultFile(self):
         """Produce a reasonable default."""
+        assert self.curframe is not None
         filename = self.curframe.f_code.co_filename
         if filename == '<string>' and self.mainpyfile:
             filename = self.mainpyfile
@@ -1949,6 +1953,9 @@ def do_debug(self, arg):
         if not arg:
             self._print_invalid_arg(arg)
             return
+        if not self.curframe:
+            self.error('No current frame.')
+            return
         self.stop_trace()
         globals = self.curframe.f_globals
         locals = self.curframe.f_locals