Fix pwn constgrep when it matches a non-constant type

This commit fixes the following issue:

```
root@pwndbg:~# pwn constgrep a
Traceback (most recent call last):
  File "/usr/local/bin/pwn", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.10/dist-packages/pwnlib/commandline/main.py", line 58, in main
    commands[args.command](args)
  File "/usr/local/lib/python3.10/dist-packages/pwnlib/commandline/constgrep.py", line 110, in main
    for _, k in sorted(out):
TypeError: '<' not supported between instances of 'Constant' and 'type'
```

Note that it was caused because of the following type object being matched and fetched from the module object:

```
ipdb> out[25:27]
[(Constant('CS', 0xd), 'CS'), (<class 'pwnlib.constants.constant.Constant'>, 'Constant')]
ipdb> sorted(out[24:27])
*** TypeError: '<' not supported between instances of 'type' and 'Constant'
```

Author: disconnect3d

pwnlib/commandline/constgrep.py

@@ -91,9 +91,13 @@ def main(args):
             if not matcher.search(k):
                 continue
 
+            # Check if the value has proper type
+            val = getattr(mod, k)
+            if not isinstance(val, pwnlib.constants.constant.Constant):
+                continue
+
             # Check the constant
             if constant is not None:
-                val = getattr(mod, k)
                 if args.mask_mode:
                     if constant & val != val:
                         continue
@@ -102,7 +106,7 @@ def main(args):
                         continue
 
             # Append it
-            out.append((getattr(mod, k), k))
+            out.append((val, k))
             maxlen = max(len(k), maxlen)
 
         # Output all matching constants