Fix possible loss of data in `mosquitto_pub -l` when sending multiple long lines.

Closes #2078. Thanks to Ysincit.

ChangeLog.txt

@@ -1,3 +1,7 @@
+Clients:
+- Fix possible loss of data in `mosquitto_pub -l` when sending multiple long
+  lines. Closes #2078.
+
 1.6.13 - 2021-02-04
 ===================
 

client/pub_client.c

@@ -262,7 +262,7 @@ int pub_stdin_line_loop(struct mosquitto *mosq)
 					break;
 				}else{
 					line_buf_len += 1024;
-					pos += 1023;
+					pos += read_len-1;
 					read_len = 1024;
 					buf2 = realloc(line_buf, (size_t )line_buf_len);
 					if(!buf2){