config_file: handle missing quotation marks in section header

When parsing a section header we expect something along the
format of '[section "subsection"]'. When a section is
mal-formated and is entirely missing its quotation marks we catch
this case by observing that `strchr(line, '"') - strrchr(line,
'"') = NULL - NULL = 0` and error out. Unfortunately, the error
message is misleading though, as we state that we are missing the
closing quotation mark while we in fact miss both quotation
marks.

Improve the error message by explicitly checking if the first
quotation mark could be found and, if not, stating that quotation
marks are completely missing.

src/config_file.c

@@ -1032,6 +1032,11 @@ 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;
+	}
+
 	last_quote = strrchr(line, '"');
 	quoted_len = last_quote - first_quote;