Fix NameError on GDB 7.7

pwndbg/events.py

@@ -56,9 +56,14 @@ registered = {gdb.events.exited: [],
               gdb.events.cont: [],
               gdb.events.new_objfile: [],
               gdb.events.stop: [],
-              gdb.events.start: [],
-              gdb.events.memory_changed: [],
-              gdb.events.register_changed: []}
+              gdb.events.start: []}
+
+# GDB 7.9 and above only
+try:
+    registered[gdb.events.memory_changed] = []
+    registered[gdb.events.register_changed] = []
+except NameError:
+    pass
 
 class Pause(object):
     def __enter__(self, *a, **kw):