fix: Process.exists() returns True when tool is missing due to operator precedence bug

`not stdout == stderr == ''` is evaluated as `not (stdout == stderr == '')`
which returns True when stderr is non-empty (e.g. "service not found"),
causing wifite to believe the tool exists and then crash with FileNotFoundError.

Replace with `bool(stdout)` — which is non-empty only when `which` actually
found the binary.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: vlkumiloslearning

wifite/util/process.py

@@ -204,7 +204,7 @@ def exists(program):
         stdout = p2.stdout().strip()
         stderr = p2.stderr().strip()
 
-        exist = not stdout == stderr == ''
+        exist = bool(stdout)
         if Configuration.initialized:
             Configuration.existing_commands.update({program: exist})
         return exist