ipdb: fix ULE when running 'context' without argument

IPython/core/debugger.py

@@ -815,9 +815,9 b' class Pdb(OldPdb):'
             new_context = int(context)
             if new_context <= 0:
                 raise ValueError()
+            self.context = new_context
         except ValueError:
             self.error("The 'context' command requires a positive integer argument.")
-        self.context = new_context
 
 
 class InterruptiblePdb(Pdb):