Fixed a crash that caused formatting failure in readlink when qemu returns None as the pid. (#1644)

* Fixed a crash that caused formatting failure in readlink when qemu returns None as the pid.

* Formated code

* Update pwndbg/arguments.py

* Assign a pid prevent repetitive calls.

* Format it

* remove the warning

* add a comment about the senario that PID is None in qemu

---------

Co-authored-by: Disconnect3d <dominik.b.czarnota@gmail.com>

pwndbg/arguments.py

@@ -221,9 +221,12 @@ def format_args(instruction):
 
         # Enhance args display
         if arg.name == "fd" and isinstance(value, int):
-            path = pwndbg.gdblib.file.readlink("/proc/%d/fd/%d" % (pwndbg.gdblib.proc.pid, value))
-            if path:
-                pretty += " (%s)" % path
+            # Cannot find PID of the QEMU program: perhaps it is in a different pid namespace or we have no permission to read the QEMU process' /proc/$pid/fd/$fd file.
+            pid = pwndbg.gdblib.proc.pid
+            if pid is not None:
+                path = pwndbg.gdblib.file.readlink("/proc/%d/fd/%d" % (pid, value))
+                if path:
+                    pretty += " (%s)" % path
 
         result.append("%-10s %s" % (N.argument(arg.name) + ":", pretty))
     return result