Fix a problem when the library load failed

The error was:
Exception ignored in: <function PyKCS11Lib.__del__ at 0x10dfa1bc0>
Traceback (most recent call last):
  File ".../PyKCS11/PyKCS11/__init__.py", line 478, in __del__
KeyError: '/path/to/invalid/lib.so'

PyKCS11/__init__.py

@@ -474,6 +474,10 @@ def __del__(self):
             if not hasattr(self, "pkcs11dll_filename"):
                 return
 
+            # in case the load failed
+            if self.pkcs11dll_filename not in PyKCS11Lib._loaded_libs:
+                return
+
             # decrease user number
             PyKCS11Lib._loaded_libs[self.pkcs11dll_filename]["nb_users"] -= 1