Silence -Wnull-dereference warning in generated CTF parser code

Silence -Wnull-dereference warning in generated CTF parser code

Building Babeltrace on amd64 with -O3 on Ubuntnu 18.04 (gcc
7.4.0-1ubuntu1~18.04.1), I see:

  make[1]: Entering directory '/home/smarchi/build/babeltrace-opt/src/plugins/ctf/common/metadata'
    CC       libctf_parser_la-lexer.lo
  /home/smarchi/src/babeltrace/src/plugins/ctf/common/metadata/lexer.c: In function ‘yyrestart’:
  /home/smarchi/src/babeltrace/src/plugins/ctf/common/metadata/lexer.c:1997:20: error: potential null pointer dereference [-Werror=null-dereference]
    b->yy_fill_buffer = 1;
    ~~~~~~~~~~~~~~~~~~^~~

This is code generated by flex, there's not much we can do, so silence
the warning for the helper library that contains the lexer/parser.

Change-Id: I6698a73f50e88cb75b94ca80deec0f3a9556c4bf
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2895
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>