config_file: handle errors other than OOM while parsing section headers

The current code in `parse_section_header_ext` is only prepared to
properly handle out-of-memory conditions for the `git_buf` structure.
While very unlikely and probably caused by a programming error, it is
also possible to run into error conditions other than out-of-memory
previous to reaching the actual parsing loop. In these cases, we will
run into undefined behavior as the `rpos` variable is only initialized
after these triggerable errors, but we use it in the cleanup-routine.

Fix the issue by unifying the function's cleanup code with an
`end_error` section, which will not use the `rpos` variable.

Author: pks-t

src/config_file.c

@@ -1027,23 +1027,23 @@ static int parse_section_header_ext(struct reader *reader, const char *line, con
 	first_quote = strchr(line, '"');
 	if (first_quote == NULL) {
 		set_parse_error(reader, 0, "Missing quotation marks in section header");
-		return -1;
+		goto end_error;
 	}
 
 	last_quote = strrchr(line, '"');
 	quoted_len = last_quote - first_quote;
 
 	if (quoted_len == 0) {
 		set_parse_error(reader, 0, "Missing closing quotation mark in section header");
-		return -1;
+		goto end_error;
 	}
 
 	GITERR_CHECK_ALLOC_ADD(&alloc_len, base_name_len, quoted_len);
 	GITERR_CHECK_ALLOC_ADD(&alloc_len, alloc_len, 2);
 
 	if (git_buf_grow(&buf, alloc_len) < 0 ||
 	    git_buf_printf(&buf, "%s.", base_name) < 0)
-		goto end_parse;
+		goto end_error;
 
 	rpos = 0;
 
@@ -1059,8 +1059,7 @@ static int parse_section_header_ext(struct reader *reader, const char *line, con
 		switch (c) {
 		case 0:
 			set_parse_error(reader, 0, "Unexpected end-of-line in section header");
-			git_buf_free(&buf);
-			return -1;
+			goto end_error;
 
 		case '"':
 			goto end_parse;
@@ -1070,8 +1069,7 @@ static int parse_section_header_ext(struct reader *reader, const char *line, con
 
 			if (c == 0) {
 				set_parse_error(reader, rpos, "Unexpected end-of-line in section header");
-				git_buf_free(&buf);
-				return -1;
+				goto end_error;
 			}
 
 		default:
@@ -1083,10 +1081,8 @@ static int parse_section_header_ext(struct reader *reader, const char *line, con
 	} while (line + rpos < last_quote);
 
 end_parse:
-	if (git_buf_oom(&buf)) {
-		git_buf_free(&buf);
-		return -1;
-	}
+	if (git_buf_oom(&buf))
+		goto end_error;
 
 	if (line[rpos] != '"' || line[rpos + 1] != ']') {
 		set_parse_error(reader, rpos, "Unexpected text after closing quotes");
@@ -1096,6 +1092,11 @@ static int parse_section_header_ext(struct reader *reader, const char *line, con
 
 	*section_name = git_buf_detach(&buf);
 	return 0;
+
+end_error:
+	git_buf_free(&buf);
+
+	return -1;
 }
 
 static int parse_section_header(struct reader *reader, char **section_out)