Fix a string parsing bug when validating extensions from the configuration

As builtin extensions are evaluated in the latter half of `check_valid_extension`, a string `cfg` is concatenated with the static string 'extension.' and the value from `builtin_extension`, before being compared with the configured value. This string is not being cleared while iterating through the names of the extensions. Because there is currently only one extension ('noop'), the bug was never noticible.

This patch corrects the behavior by clearing the string on each iteration, as is done in the first block.

src/libgit2/repository.c

@@ -1462,6 +1462,7 @@ static int check_valid_extension(const git_config_entry *entry, void *payload)
 	}
 
 	for (i = 0; i < ARRAY_SIZE(builtin_extensions); i++) {
+		git_str_clear(&cfg);
 		extension = builtin_extensions[i];
 
 		if ((error = git_str_printf(&cfg, "extensions.%s", extension)) < 0)