Update lib_mysqludf_sys.c

[wa5620]

There is big bug for sys_eval which could make crash for mysql.
in char* sys_eval()
Parameter char* result is a fixed char[256] provied by framework. can not be realloc.
We could alloc bigger memory by ourself and free it at sys_eval_deinit().

[R31N3]

I tried this fork on my server, now it doesn't crash, but there's no result too, just NULL. I tried something like:
SELECT sys_eval('curl https://google.com') as result;

Without your fix it worked currec half the time, sometimes worked, sometimes crashed.

[wa5620]

I tried this fork on my server, now it doesn't crash, but there's no result too, just NULL. I tried something like:
SELECT sys_eval('curl https://google.com') as result;

Without your fix it worked currec half the time, sometimes worked, sometimes crashed.

May you try debug little bit, to see where the problem is.
The code works fine on my server Redhat73 mysql5.7 for a long time.