Fix xdg folder permission check for cases when privileges were dropped

If privileges for scapy were dropped, but username remain unchanged,
path.exist() would trigger an exception. Fix that by moving whole
if statement under try-except.

Fixes secdev#4618

scapy/main.py

@@ -68,17 +68,18 @@
 def _probe_xdg_folder(var, default, *cf):
     # type: (str, str, *str) -> Optional[pathlib.Path]
     path = pathlib.Path(os.environ.get(var, default))
-    if not path.exists():
-        # ~ folder doesn't exist. Create according to spec
-        # https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
-        # "If, when attempting to write a file, the destination directory is
-        # non-existent an attempt should be made to create it with permission 0700."
-        try:
+    try:
+        if not path.exists():
+            # ~ folder doesn't exist. Create according to spec
+            # https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
+            # "If, when attempting to write a file, the destination directory is
+            # non-existent an attempt should be made to create it with permission 0700."
             path.mkdir(mode=0o700, exist_ok=True)
-        except Exception:
-            # There is a gazillion ways this can fail. Most notably,
-            # a read-only fs.
-            return None
+    except Exception:
+        # There is a gazillion ways this can fail. Most notably, a read-only fs or no
+        # permissions to even check for folder to exist (e.x. privileges were dropped
+        # before scapy was started).
+        return None
     return path.joinpath(*cf).resolve()