config_parse: always sanitize out-parameters in `parse_variable`

The `parse_variable` function has two out parameters `var_name` and
`var_value`. Currently, those are not being sanitized to `NULL`. when.
any error happens inside of the `parse_variable` function. Fix that.
While at it, the coding style is improved to match our usual coding
practices more closely.

src/config_parse.c

@@ -404,22 +404,21 @@ static int parse_name(
 static int parse_variable(git_config_parser *reader, char **var_name, char **var_value)
 {
 	const char *value_start = NULL;
-	char *line;
-	int quote_count;
+	char *line = NULL, *name = NULL, *value = NULL;
+	int quote_count, error;
 	bool multiline;
 
+	*var_name = NULL;
+	*var_value = NULL;
+
 	git_parse_advance_ws(&reader->ctx);
 	line = git__strndup(reader->ctx.line, reader->ctx.line_len);
-	if (line == NULL)
-		return -1;
+	GITERR_CHECK_ALLOC(line);
 
 	quote_count = strip_comments(line, 0);
 
-	/* If there is no value, boolean true is assumed */
-	*var_value = NULL;
-
-	if (parse_name(var_name, &value_start, reader, line) < 0)
-		goto on_error;
+	if ((error = parse_name(&name, &value_start, reader, line)) < 0)
+		goto out;
 
 	/*
 	 * Now, let's try to parse the value
@@ -428,30 +427,34 @@ static int parse_variable(git_config_parser *reader, char **var_name, char **var
 		while (git__isspace(value_start[0]))
 			value_start++;
 
-		if (unescape_line(var_value, &multiline, value_start, 0) < 0)
-			goto on_error;
+		if ((error = unescape_line(&value, &multiline, value_start, 0)) < 0)
+			goto out;
 
 		if (multiline) {
 			git_buf multi_value = GIT_BUF_INIT;
-			git_buf_attach(&multi_value, *var_value, 0);
+			git_buf_attach(&multi_value, value, 0);
 
 			if (parse_multiline_variable(reader, &multi_value, quote_count) < 0 ||
-				git_buf_oom(&multi_value)) {
+			    git_buf_oom(&multi_value)) {
+				error = -1;
 				git_buf_dispose(&multi_value);
-				goto on_error;
+				goto out;
 			}
 
-			*var_value = git_buf_detach(&multi_value);
+			value = git_buf_detach(&multi_value);
 		}
 	}
 
-	git__free(line);
-	return 0;
+	*var_name = name;
+	*var_value = value;
+	name = NULL;
+	value = NULL;
 
-on_error:
-	git__free(*var_name);
+out:
+	git__free(name);
+	git__free(value);
 	git__free(line);
-	return -1;
+	return error;
 }
 
 int git_config_parse(