only use error message if giterr_last() is not null

exptected errors in libgit2 must not set a git_error.

src/pygit2/error.c

@@ -113,6 +113,9 @@ PyObject* Error_set_str(int err, const char *str)
     }
 
     error = giterr_last();
+    if (error == NULL) //exptected error - no error msg set
+        return PyErr_Format(Error_type(err), "%s", str);
+
     return PyErr_Format(Error_type(err), "%s: %s", str, error->message);
 }