cmdline: fix compilation with -Og

cmdline: fix compilation with -Og

The compilation with gcc-6.3.0 and EXTRA_CFLAGS=-Og gives the following
error:

  CC cmdline_parse.o
  cmdline_parse.c: In function ‘match_inst’:
  cmdline_parse.c:227:5: error: ‘token_p’ may be used uninitialized in
                         this function [-Werror=maybe-uninitialized]
    if (token_p) {
       ^

This is a false positive, gcc is not able to see that we always go in
the loop at least once, initializing token_p.

Fix the warning by initializing token_p to NULL.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>