config_parse: refactor error handling when parsing multiline variables

The current error handling for the multiline variable parser is a bit
fragile, as each error condition has its own code to clear memory.
Instead, unify error handling as far as possible to avoid this
repetitive code. While at it, make use of `GITERR_CHECK_ALLOC` to
correctly handle OOM situations and verify that the buffer we print into
does not run out of memory either.

(cherry picked from commit bc63e1ef)

src/config_file.c

@@ -1347,44 +1347,49 @@ done:
 
 static int parse_multiline_variable(struct reader *reader, git_buf *value, int in_quotes)
 {
-	char *line = NULL, *proc_line = NULL;
 	int quote_count;
 	bool multiline = true;
 
 	while (multiline) {
+		char *line = NULL, *proc_line = NULL;
+		int error;
+
 		/* Check that the next line exists */
 		line = reader_readline(reader, false);
-		if (line == NULL)
-			return -1;
+		GITERR_CHECK_ALLOC(line);
 
-		/* We've reached the end of the file, there is no continuation.
+		/*
+		 * We've reached the end of the file, there is no continuation.
 		 * (this is not an error).
 		 */
 		if (line[0] == '\0') {
-			git__free(line);
-			return 0;
+			error = 0;
+			goto out;
 		}
 
+		/* If it was just a comment, pretend it didn't exist */
 		quote_count = strip_comments(line, !!in_quotes);
+		if (line[0] == '\0')
+			goto next;
 
-		/* If it was just a comment, pretend it didn't exist */
-		if (line[0] == '\0') {
-			git__free(line);
-			in_quotes = quote_count;
-			continue;
-		}
+		if ((error = unescape_line(&proc_line, &multiline,
+					   line, in_quotes)) < 0)
+			goto out;
 
-		if (unescape_line(&proc_line, &multiline, line, in_quotes) < 0) {
-			git__free(line);
-			return -1;
-		}
-		/* add this line to the multiline var */
+		/* Add this line to the multiline var */
+		if ((error = git_buf_puts(value, proc_line)) < 0)
+			goto out;
 
-		git_buf_puts(value, proc_line);
+next:
 		git__free(line);
 		git__free(proc_line);
-
 		in_quotes = quote_count;
+		continue;
+
+out:
+		git__free(line);
+		git__free(proc_line);
+		return error;
 	}
 
 	return 0;