Fix exception when pwndbg is sourced after attach

This commit fixes an exception when pwndbg is sources after gdb is
attached to a process.

Below log is with `set exception-verbose on` and `set exception-debugger
on` hardcoded into the sources. We can see that:
1. We get an immediate exception
2. The `pwndbg.arch.current` is set incorrectly to `i386`
3. The real arch is `i386:x86-64`

```
$ gdb -q -p $(pidof a.out) ./a.out
Reading symbols from ./a.out...
(No debugging symbols found in ./a.out)
Attaching to program: /home/dc/example/a.out, process 415704
Reading symbols from /lib/x86_64-linux-gnu/libc.so.6...
Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.31.so...
Reading symbols from /lib64/ld-linux-x86-64.so.2...
(No debugging symbols found in /lib64/ld-linux-x86-64.so.2)
0x00007f4bb94e7142 in __GI___libc_read (fd=0, buf=0x55ee576fb6b0, nbytes=1024)
at ../sysdeps/unix/sysv/linux/read.c:26
26      ../sysdeps/unix/sysv/linux/read.c: No such file or directory.
(gdb) source /home/dc/tools/pwndbg/gdbinit.py
pwndbg: loaded 195 commands. Type pwndbg [filter] for a list.
pwndbg: created $rebase, $ida gdb functions (can be used with print/break)
Traceback (most recent call last):
File "/home/dc/tools/pwndbg/pwndbg/commands/__init__.py", line 130, in __call__
return self.function(*args, **kwargs)
File "/home/dc/tools/pwndbg/pwndbg/commands/__init__.py", line 221, in _OnlyWhenRunning
return function(*a, **kw)
File "/home/dc/tools/pwndbg/pwndbg/commands/context.py", line 269, in context
result[target].extend(func(target=out,
File "/home/dc/tools/pwndbg/pwndbg/commands/context.py", line 350, in context_regs
regs = get_regs()
File "/home/dc/tools/pwndbg/pwndbg/commands/context.py", line 399, in get_regs
m = ' ' * len(change_marker) if reg not in changed else C.register_changed(change_marker)
TypeError: argument of type 'NoneType' is not iterable

If that is an issue, you can report it on https://github.com/pwndbg/pwndbg/issues
(Please don't forget to search if it hasn't been reported before)
To generate the report and open a browser, you may run `bugreport --run-browser`
PS: Pull requests are welcome
> /home/dc/tools/pwndbg/pwndbg/commands/context.py(399)get_regs()
-> m = ' ' * len(change_marker) if reg not in changed else C.register_changed(change_marker)
(Pdb) print(changed)
None
(Pdb) print(reg)
eax
(Pdb) print(pwndbg.arch.current)
i386
(Pdb) print(gdb.execute('show arch', to_string=True))
The target architecture is set automatically (currently i386:x86-64)

(Pdb)
```

pwndbg/prompt.py

@@ -20,7 +20,7 @@ hint_lines = (
 for line in hint_lines:
     print(message.prompt('pwndbg: ') + message.system(line))
 
-cur = (gdb.selected_inferior(), gdb.selected_thread())
+cur = None
 
 
 def prompt_hook(*a):
@@ -30,7 +30,7 @@ def prompt_hook(*a):
     new = (gdb.selected_inferior(), gdb.selected_thread())
 
     if cur != new:
-        pwndbg.events.after_reload(start=False)
+        pwndbg.events.after_reload(start=cur is None)
         cur = new
 
     if pwndbg.proc.alive and pwndbg.proc.thread_is_stopped: