Fix: ctf: free all scopes of ctf_scanner

Fix: ctf: free all scopes of ctf_scanner

When parsing using a ctf_scanner does not complete, because an error is
encountered, ctf_scanner_free does not properly free all the scopes
allocated by and held by ctf_scanner.

This can be observed on an ASan-enabled build of Babeltrace.  Create a
test/metadata file with:

    /* CTF 1.8 */

    trace {}

and run:

    $ LIBBABELTRACE2_NO_DLCLOSE=1 ./src/cli/babeltrace2 query src.ctf.fs metadata-info -p 'path=test'

I get:

    Direct leak of 16 byte(s) in 1 object(s) allocated from:
        #0 0x7ffff7677459 in __interceptor_malloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:145
        #1 0x7ffff35605b9 in push_scope /home/simark/src/babeltrace/src/plugins/ctf/common/metadata/parser.y:296
        #2 0x7ffff356726f in yyparse /home/simark/src/babeltrace/src/plugins/ctf/common/metadata/parser.y:1478
        #3 0x7ffff3563297 in ctf_scanner_append_ast /home/simark/src/babeltrace/src/plugins/ctf/common/metadata/parser.y:958
        #4 0x7ffff3598a6a in ctf_metadata_decoder_append_content /home/simark/src/babeltrace/src/plugins/ctf/common/metadata/decoder.c:277
        #5 0x7ffff3626fff in metadata_info_query /home/simark/src/babeltrace/src/plugins/ctf/fs-src/query.c:118
        #6 0x7ffff3625ac1 in ctf_fs_query /home/simark/src/babeltrace/src/plugins/ctf/fs-src/fs.c:2454
        #7 0x7ffff74a616d in bt_query_executor_query /home/simark/src/babeltrace/src/lib/graph/query-executor.c:219
        #8 0x5555555a09e2 in cli_query /home/simark/src/babeltrace/src/cli/babeltrace2-query.c:62
        #9 0x555555574fb3 in query /home/simark/src/babeltrace/src/cli/babeltrace2.c:113
        #10 0x5555555775c6 in cmd_query /home/simark/src/babeltrace/src/cli/babeltrace2.c:645
        #11 0x555555582ae7 in main /home/simark/src/babeltrace/src/cli/babeltrace2.c:2682
        #12 0x7ffff70ebb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

What happens is: while parsing, push_scope is called to allocate a new
scope and make it current (ctf_scanner::cs).  The previous current scope
becomes the parent of the new scope.  When everything goes right,
pop_scope is called as many times as push_scope is called, freeing all
the scopes created during parsing.  ctf_scanner_free doesn't need to
free any scopes in that case.  However, if parsing ends early to due to
an error, ctf_scanner_free is called on a scanner that may have some
scopes that were pushed and not popped.  If so, it needs to free them.

Change it to start freeing ctf_scanner::cs (the innermost scope) and
then go up the chain.  The last scope in that chain is
ctf_scanner::root_scope.  This one doesn't need free'ing, as its storage
is allocate within the ctf_scanner structure, but it still needs
finalizing.

Change-Id: I3a1b83af8128cd1b8a3ee235311dd41e45e29ea2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/5692
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>