BUG/MEDIUM: log-format: fix possible endless loop in parse_logformat_string()

This patch adds a missing break to end the loop in case when '%[' is not
properly closed with ']'.

The issue has been introduced with commit cd0d2ed ("MEDIUM: log-format:
make the LF parser aware of sample expressions' end").

src/log.c

@@ -683,6 +683,7 @@ int parse_logformat_string(const char *fmt, struct proxy *curproxy, struct list
 					memprintf(err, "expected ']' after '%s', but found '%c'", var, c);
 				else
 					memprintf(err, "missing ']' after '%s'", var);
+				goto fail;
 			}
 			break;