Fix `mosquitto_passwd -b` using username as password.

Only applies if if `-c` is not also used.

Closes #1949. Thanks to J. Augusto de Oliveira.

ChangeLog.txt

@@ -2,6 +2,10 @@ Broker:
 - Fix excessive CPU use on non-Linux systems when the open file limit is set
   high. Closes #1947.
 
+Apps:
+- Fix `mosquitto_passwd -b` using username as password (not if `-c` is also
+  used). Closes #1949.
+
 Build:
 - Fix `install` target when using WITH_CJSON=no. Closes #1938.
 

apps/mosquitto_passwd/mosquitto_passwd.c

@@ -505,7 +505,7 @@ int main(int argc, char *argv[])
 	}else if(batch_mode == true && idx+3 == argc){
 		password_file_tmp = argv[idx];
 		username = argv[idx+1];
-		password_cmd = argv[idx+1];
+		password_cmd = argv[idx+2];
 	}else if(batch_mode == false && idx+2 == argc){
 		password_file_tmp = argv[idx];
 		username = argv[idx+1];