Fix: lttng: free domains and channels in get_session_stats_str

Fix: lttng: free domains and channels in get_session_stats_str

When doing `lttng stop`, I get:

    Direct leak of 656 byte(s) in 1 object(s) allocated from:
        #0 0x7f970719e459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
        #1 0x7f9706eba29a in zmalloc /home/simark/src/lttng-tools/src/common/macros.h:45
        #2 0x7f9706ebbb9d in recv_sessiond_optional_data /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:494
        #3 0x7f9706ebbf9a in lttng_ctl_ask_sessiond_fds_varlen /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:596
        #4 0x7f9706eba714 in lttng_ctl_ask_sessiond_varlen_no_cmd_header /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl-helper.h:58
        #5 0x7f9706eba747 in lttng_ctl_ask_sessiond /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl-helper.h:78
        #6 0x7f9706ec4604 in lttng_list_channels /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:2262
        #7 0x55837235c4e7 in get_session_stats_str /home/simark/src/lttng-tools/src/bin/lttng/utils.c:499
        #8 0x55837235bf73 in print_session_stats /home/simark/src/lttng-tools/src/bin/lttng/utils.c:445
        #9 0x55837231cc12 in stop_tracing /home/simark/src/lttng-tools/src/bin/lttng/commands/stop.c:138
        #10 0x55837231d062 in cmd_stop /home/simark/src/lttng-tools/src/bin/lttng/commands/stop.c:229
        #11 0x55837235e63e in handle_command /home/simark/src/lttng-tools/src/bin/lttng/lttng.c:237
        #12 0x55837235f0f2 in parse_args /home/simark/src/lttng-tools/src/bin/lttng/lttng.c:426
        #13 0x55837235f3f4 in main /home/simark/src/lttng-tools/src/bin/lttng/lttng.c:475
        #14 0x7f9706adcb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

    Direct leak of 308 byte(s) in 1 object(s) allocated from:
        #0 0x7f970719e459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
        #1 0x7f9706eba29a in zmalloc /home/simark/src/lttng-tools/src/common/macros.h:45
        #2 0x7f9706ebbb9d in recv_sessiond_optional_data /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:494
        #3 0x7f9706ebbf9a in lttng_ctl_ask_sessiond_fds_varlen /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:596
        #4 0x7f9706eba714 in lttng_ctl_ask_sessiond_varlen_no_cmd_header /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl-helper.h:58
        #5 0x7f9706eba747 in lttng_ctl_ask_sessiond /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl-helper.h:78
        #6 0x7f9706ec421c in lttng_list_domains /home/simark/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:2220
        #7 0x55837235c3d3 in get_session_stats_str /home/simark/src/lttng-tools/src/bin/lttng/utils.c:484
        #8 0x55837235bf73 in print_session_stats /home/simark/src/lttng-tools/src/bin/lttng/utils.c:445
        #9 0x55837231cc12 in stop_tracing /home/simark/src/lttng-tools/src/bin/lttng/commands/stop.c:138
        #10 0x55837231d062 in cmd_stop /home/simark/src/lttng-tools/src/bin/lttng/commands/stop.c:229
        #11 0x55837235e63e in handle_command /home/simark/src/lttng-tools/src/bin/lttng/lttng.c:237
        #12 0x55837235f0f2 in parse_args /home/simark/src/lttng-tools/src/bin/lttng/lttng.c:426
        #13 0x55837235f3f4 in main /home/simark/src/lttng-tools/src/bin/lttng/lttng.c:475
        #14 0x7f9706adcb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

This is due to the get_session_stats_str function not free'ing the
results of lttng_list_channels and lttng_list_domains.  Fix that.

Change-Id: I4c200d3df41bf09bdce8eadb000abbff7fe5a751
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>