scylla-gdb.py: scylla fiber: add actual return to early return

scylla_fiber._walk() has an early return condition on the passed-in
pointer actually being a task pointer. The problem is that the actual
return statement was missing and only an error was logged. This resulted
in execution continuing and further weird errors being printed due to
the code not knowing how to handle the bad pointer.
Message-Id: <20200818144902.2357289-1-bdenes@scylladb.com>

scylla-gdb.py

@@ -2701,6 +2701,7 @@ class scylla_fiber(gdb.Command):
         this_task = self._probe_pointer(ptr, scanned_region_size, using_seastar_allocator, verbose)
         if this_task is None:
             gdb.write("Provided pointer 0x{:016x} is not an object managed by seastar or not a task pointer\n".format(ptr))
+            return this_task, []
 
         i = 0
         tptr_meta = this_task[0]