build warnings core cfg

[sergey-safarov]

CC (gcc) [kamailio]		core/crypto/shautils.o
CC (gcc) [kamailio]		core/cfg/cfg.o
CC (gcc) [kamailio]		core/cfg/cfg_ctx.o
core/cfg/cfg_ctx.c: In function 'cfg_update_defaults':
core/cfg/cfg_ctx.c:287:28: warning: the comparison will always evaluate as 'true' for the pointer operand in '(unsigned char *)&ginst->vars + (sizetype)var->offset' must not be NULL [-Waddress]
  287 |                         if(ginst->vars + var->offset) {
      |                            ^~~~~
CC (gcc) [kamailio]		core/cfg/cfg_script.o
CC (gcc) [kamailio]		core/cfg/cfg_select.o

[miconda]

Not sure about the purpose of the check, not being the developer, probably is useless, but I pushed a commit to silent the warning just to play safe for the moment.