[sr-dev] [kamailio/kamailio] kamcmd.c: memory leak (Issue #3478)

``` ==70169==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 4096 byte(s) in 15 object(s) allocated from: #0 0xffffaed756eb in malloc (/lib64/libasan.so.5+0xd56eb) #1 0x4137e7 in get_cfgvars_list /usr/src/debug/kamailio-5.6.4-4.el8.centos.aarch64/utils/kamcmd/kamcmd.c:1353 #2 0x41ea03 in main (/usr/sbin/kamcmd+0x41ea03) #3 0xffffade04383 in __libc_start_main (/lib64/libc.so.6+0x24383) #4 0x402cbb (/usr/sbin/kamcmd+0x402cbb)

SUMMARY: AddressSanitizer: 4096 byte(s) leaked in 15 allocation(s). ```