expression: initialize list of expression to silence gcc compile warning

expression: initialize list of expression to silence gcc compile warning

The helper function to translate flagcmp expression to binop expression
results in the following compile warning.

  src/expression.c: In function 'list_expr_to_binop':
  src/expression.c:1286:16: warning: 'last' may be used uninitialized [-Wmaybe-uninitialized]
  1286 |         return last;

While at it, add assert() to validate the premises where this function
can be called.

Fixes: 4d5990c92c83 ("src: transform flag match expression to binop expression from parser")
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>