parser: don't assert on scope underflows

parser: don't assert on scope underflows

close_scope() gets called from the object destructors;
imbalance can cause us to hit assert().

Before:
nft: parser_bison.y:88: close_scope: Assertion `state->scope > 0' failed.
After:
assertion3:4:7-7: Error: too many levels of nesting jump {
assertion3:5:8-8: Error: too many levels of nesting jump
assertion3:5:9-9: Error: syntax error, unexpected newline, expecting '{'
assertion3:7:1-1: Error: syntax error, unexpected end of file

Signed-off-by: Florian Westphal <fw@strlen.de>