[sr-dev] [kamailio/kamailio] ndb_redis: fix SIGSEGV in redisc_check_auth (#2834)

[Dennis]

Hi !

This issue appears when ndb_redis module connect to redis via proxy, in my case haproxy.
Haproxy keeps front socket always open, when all backend redis servers become unavailable, haproxy just send RST to front,
redisCommand function in tis case return NULL poiner, redisc_check_auth try to dereference it without NULL checking.

kamailio log:
`kernel: [7250430.508827] kamailio[21749]: segfault at 0 ip 00007f00d7bb106f sp 00007fffc523c620 error 4 in ndb_redis.so[7f00d7b94000+36000]`

gdb:
```
# gdb /usr/sbin/kamailio core
GNU gdb (Ubuntu 8.1.1-0ubuntu1) 8.1.1
Copyright (C) 2018 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /usr/sbin/kamailio...(no debugging symbols found)...done.
[New LWP 21752]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `/usr/sbin/kamailio -P /run/kamailio/kamailio.pid -f /etc/kamailio/kamailio.cfg'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007f00d7bb106f in redisc_check_auth () from /usr/lib64/kamailio/modules/ndb_redis.so
(gdb) bt
#0  0x00007f00d7bb106f in redisc_check_auth () from /usr/lib64/kamailio/modules/ndb_redis.so
#1  0x00007f00d7b9937a in redisc_init () from /usr/lib64/kamailio/modules/ndb_redis.so
#2  0x00007f00d7bb4300 in print_redis_reply () from /usr/lib64/kamailio/modules/ndb_redis.so
#3  0x0000561825796935 in ?? ()
#4  0x0000561825796550 in ?? ()
#5  0x0000561825796550 in ?? ()
#6  0x0000561825796550 in ?? ()
#7  0x0000561825796550 in ?? ()
#8  0x0000561825796550 in ?? ()
#9  0x0000561825796550 in ?? ()
#10 0x0000561825796550 in ?? ()
#11 0x0000561825796550 in ?? ()
#12 0x0000561825796550 in ?? ()
#13 0x0000561825796550 in ?? ()
#14 0x0000561825796550 in ?? ()
#15 0x0000561825796550 in ?? ()
#16 0x0000561825796550 in ?? ()
#17 0x000056182579722c in init_child ()
#18 0x00005618256f4520 in fork_process ()
#19 0x00005618255cfbf3 in main_loop ()
#20 0x00005618255dd095 in main ()
```


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You can view, comment on, or merge this pull request online at:

  https://github.com/kamailio/kamailio/pull/2834

-- Commit Summary --

  * ndb_redis: fix SIGSEGV in redisc_check_auth

-- File Changes --

    M src/modules/ndb_redis/redis_client.c (4)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/2834.patch
https://github.com/kamailio/kamailio/pull/2834.diff

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